Helios Paint is a free paint and image editing program that works on Mac, Windows, Linux and Unix, so you should be covered no matter what the computer. It comes with all the usual nice bits, brushes, filters and the like, and will handle most image file formats including JPEG, GIF and TIFF. Worth a look.
HeliosPaint is a freeware paint program for creating and editing images. It runs on Microsoft Windows, Mac OS X, Linux and Unix and is written in Sun Microsystem’s Java programming language.