The Gimp isn't the only open source image editor in town. Seashore may be quite basic in its functionality, but it's pleasant to use and the source code is freely available. Seashore employs many of the tools you're familiar with, like the lasso, marquee tool, smudge, erase, position, paint and more. Plus, there are several brushes and textures to choose from. Export options include JPEG, JPEG 2000, PNG, TIFF and XCF (Gimp). Seashore also supports layers and alpha channel editing.
Seashore is free, universal and requires Mac OS 10.3 or later.