Gimp the most popular Open Source image editor just released the 2.4 version. This new version comes with a bundle of new features and improvements, like Improved display when zooming in or out, better Scalable Brushes and Selection Tools, lens distortion filters and a lot more.
Most Linux distributions come with Gimp as a pre-installed, for example Ubuntu 7.10 -Gutsy Gibbon comes with a pre-installed Gimp 2.4 version. An update could be enough.
Gimp is also available in the Fedora and openSUSE repositories and for download as source code.
For beginners the Gimp User Manual is a recommend starting point.