Few people will argue that you can’t go wrong with Gimp, if you’re interested in a mature and feature-packed image editor. It’s been around for years and boasts of a vast user community. The project is also incredibly well documented, and the subject of several books, and you can also find tutorials and videocasts online.
Krita is also an excellent option, but would appeal more to budding artists. Although the program has been primarily designed for painting and animation, you could still use it for performing some basic image editing. If you spend a lot of time with raw images, you’ll appreciate
RawTherapee. Its non-destructive image editing and batch processing abilities make it popular with pros. Although it hasn’t seen a new release in a few years, UFRaw is another option, especially if you prefer working from the command line, as its batch processing can be invoked from the terminal itself.