How to resize pictures for email

There is an old joke about a recipe to make rabbit stew. It starts with, "first catch your rabbit".

If you already have your pictures and are ready to reduce them then we can get started, however if need to get the pictures out of an email first, click here.

These are the steps to reduce an image's size to make it suitable to send in email:

  1. load the picture into Gimp
  2. rescale its size
  3. save the picture

  4. extra improvements - these aren't necessary, but can help improve a picture.

Load a picture into Gimp

Remember that you can undo anything in Gimp. Under the main toolbox there is the "Undo" tab. It has a curved yellow arrow on it. Clicking this displays a list of all the changes you've made. At any time you can move backwards and forwards in that history of changes.

Unfortunately the history list doesn't branch though, so that if you make a change, then undo it, and make another change, you'll find that the earlier change that was undone is now lost and you can no longer go back to it. You can get around this by saving versions of the picture when you are fiddling and experimenting so you can always re-find happy accidents.

You can load a picture into Gimp various different ways. You can drop the picture's icon onto the Gimp icon or right-click the picture's icon and choose Gimp in the pop-up menu. Or you could start Gimp and drop the picture icon onto Gimp's tool area (that's the part of Gimp with about 20 tool icons).

Rescale the picture's size

Once it's in Gimp look at the menu bar at the top of the image and pull down the "View" menu and choose "Scale Image...". If you let your mouse linger over that menu item for a second or two a hint will display telling you a little more about what that item does, with the further suggestion to press the F1 key for more help. (If you press the F1 key while the mouse is still over that menu item Gimp will open the manual with the help page about that particular item!)

From now on I'll refer to such menu choices like this:
Image Scale Image...

This presents you with a window in which you get to choose what size you want the final image to be.

It opens with the most useful items set for you. Notice that the width of the picture in pixels is already highlighted (in blue in my example; it may be a different highlight color in yours). Just type in what width you want the picture to be -- you don't need to click anything to select it because it is already selected for you. A good width for email depends upon what speed of internet access the person has and what speed of computer they have. A fairly good guide is no more than perhaps 800 pixels. But... if the picture is taller than it is wide, as my example is here, then its height is more important. In that case don't worry about the width and select the height instead. A good maximum height can be something like 700 pixels or perhaps even less, if you don't lose too much detail -- such things are always a trade-off, with you having to juggle the filesize against the clarity of the picture.

You will notice that when you press "Enter" on your keyboard, or click anywhere else on that "Scale Image" window that the other of the pair of values will change. If you altered the width then height will automatically change in proportion. Likewise if you altered height the width will change accordingly. That is what the little chain symbol next to the width and height values does. Clicking on it "breaks" the chain so that they are no longer linked together and you can alter them independently, but you don't want that. If you accidentally do unlink them click the little chain again and it will recouple them.

You have a choice of using "pixels" for the measuring unit. Leave it at that for now. In some cases "percent" can be useful, but not in this particular case, and "inches", "millimeters", "points", and "picas" are only useful for printing.

Quality is best left at "Cubic". On the rare occasion you wish to enlarge something you will get better results with "Sinc (Lanczos 3)" unless it is intended to be blocky in which case you might want to choose "none". But as I say, for most purposes "Cubic" works best. It does a very good job of averaging groups of pixels when shrinking them down to a lesser number of pixels -- a surprisingly tricky problem.

So just click the "Scale" button after you've entered the new width or height and you are done.

Save the picture

Don't forget to save the final image.
File Save As...

Some notes on file format

Most photographs should probably be saved using JPEG (.jpg) file format. It is a system developed by the Joint Photographic Experts Group, hence the name. This is a very efficient format that saves even quite large pictures as suprisingly small filesize. It does this by throwing information away in a very tricky fashion, taking advantage of the fact that we're not good at seeing slight variations in very light or very dark parts of images. So it throws a lot of that data away -- you'll probably not notice. If you choose a low number for the quality of the compression then it will save your big picture as a very small file by discarding lots of information -- perhaps even from parts of the image that you will notice. If you choose a very high number then it will lose very little data, but your filesize will be accordingly larger. A good choice for most situations seems to be between 80 and 95. I usually use 85.

The reason why it is important to know how JPEG works becomes apparent if you want to enhance a file, by perhaps bringing out detail on a person's face who is deep in shadow. If you are working from a JPEG picture to start with then you'll find that all the detail in the shadows have already been thrown away by JPEG compression. If you want to play with a picture then it is best to use one of the "non-lossy" formats, like PNG or TIFF, then when you are satisfied with the final result save that as JPEG.

Needless to say it is no use re-saving a JPEG file as PNG in order to work on it because the damage is already done when it was first compressed.

I should point out that the formats are determined by how the picture is stored -- the extension (.jpg or .png or .tif or .gif) is just an indicator for the human to let them know what format the file is stored as. If you change the ".jpg" at the end of a file to ".png" it will still be a JPEG file. The computer will know this, but it will confuse the human by making them think it isn't.

JPEG files are stored by breaking the image down into ever smaller squares until pixels in that square differ by less than a certain amount, then storing that square as a single color. There is a little more to it than that, but that is the essence.

PNG files look at pixels as horizontal lines of different colored dots, similar to how we read text as horizontal lines of letters. PNG encoding looks for groups of dots that have the same pattern and record them as a code number. This is why PNG is very good at storing diagrams as very small files, but storing photographs as PNG results in enormous filesizes -- there are not likely to be many repeating patterns in a photograph. Many other file formats use similar techniques to PNG encoding, but PNG is one of the best non-lossy formats, probably because it was designed so recently and was able to take into account a lot of new developments.

getting your pictures out of email

If the original picture is an attachment to an email then you'll need to save it to somewhere convenient on your computer first. In Seamonkey, just right-click one of the attached pictures in the little attachment window and choose from the pop-up menu "Save all". That will ripple through all the attachments in that email and save them to somewhere that you choose.

If the image is "in-lined" into the email without actually being an attachment then you'll need to go to each image, right-click it and choose "Save Image".

Now you have the pictures somewhere on your computer that you can edit them. I prefer to use Gimp for this. You can use Irfanview, but I can't remember the steps for it so am unable to give them here. However it is pretty-much the same for most programs.

Now that you have your picture return to the main section.

Extra improvements

When you reduce a picture you unavoidably lose information, however you can use various enhancement techniques to minimise the loss.

Filters Enhance Sharpen...

Often, choosing from the "Filters" menu, in the "Enhance" sub-menu, the item "Sharpen" can do wonders to the slight blur introduced to resized images. It will open a little window with a preview onto you image which you can pan around to the most important part so you can see what effect the sharpen will have before actually using it. I recommend not using more than about 20 for sharpening. On some blurry pictures you might be able to go up to maybe 50, but beyond that any artifacts in the picture start to stand out and ruin the picture.

Colors Auto White Balance

Another couple of things that can lift a picture are on the "Colors" menu. Sometimes choosing from the "Auto" sub-menu the "White Balance" item can fix the color of a picture that has a lot of reflected light bouncing off a colored object nearby, distorting the color of the picture.

Colors Levels...

Sometimes you can dramatically improve a picture by choosing from the "Colors" menu the "Levels..." item. This shows a window that tells you about the various levels in the picture. The most useful part is the "Auto" button. Sometimes that can perform wonders. Sometimes it does almost nothing. Sometimes the result is really yucky. Remember you can always undo.

Colors Curves...

This is one of my favorite tools. I could write volumes about this. It can occasionally perform miracles upon images. Unfortunately, pretty-much the only way to find out how to use it is to fiddle. Give yourself a spare hour to play with it and you'll find that what defies easy explanation becomes, after a while, quite intuitive. Basically it lets you alter the lightness and darkness of pixels depending on what their existing lightness and darkness is. This sounds like gobbledegook until you fiddle with it for a while and then you'll find that it somehow becomes clear what is happening. Every picture will require tweaking a different way, but the benefit can be truly amazing.

Colors Hue-Saturation...

This one is divided into two parts. The top part, consists of a strange circle of colors with an overlap slider below, and, although it doesn't look like it, the "Hue" slider below that. You'll almost never want to play with the that part when working on photos, though it can produce fantastic results for diagrams and artworks. The really useful part is the pair of sliders at the bottom. "Lightness" does exactly as it says. "Saturation" boosts the amount of color in an image or drains it way towards greyscale. If you are not happy with the result you don't need to undo -- just "Cancel" instead of "OK".

There are hundreds of other wonderful things that Gimp can do, but those are the ones I find most useful. The best way to find nice functions in Gimp is to experiment and to read the manual -- remember it is always just a press of the F1 key away.