Free and Open Source Art Software

These programs are capable of saving your pictures as compressed file formats, which means they'll be smaller file sizes for use on the web. Admittedly, a bit of quality is lost when you save a picture as a JPEG, GIF, or PNG, but it's worth it, especially since it probably won't destroy the picture if you know what you're doing.

Add more if you know of any that aren't here yet.

GIMP: http://www.gimp.org/
Platforms: Windows, Mac, Unix
Formats:
Review pending.

OptiPNG: http://optipng.sourceforge.net/
Platforms: Various (source code), Windows (binary)
Formats: PNG
OptiPNG is a small command-line utility used to losslessly recompress PNG images so that they are as small as possible. It can achieve savings of as much as 40% on some images, notably those exported by Photoshop for Windows.

pngquant: http://www.libpng.org/pub/png/apps/pngquant.html
Platforms: Linux, Windows (source & binary)
Formats: PNG
pngquant serves a single use: it converts true-colour PNG images into paletted (256-colour or less) PNG images, applying dithering where appropriate. Like OptiPNG above it is a command-line tool, but there is a graphical interface available (http://jedisthlm.com/2006/03/16/manfred-a-pngquant-gui/). This tool becomes especially useful when dealing with semi-transparent images, as most image-editing software (all flavours of Photoshop included) do not deal with paletted transparent images at all.

(add any more you know of here)

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