I was going to recommend Teal, but the developer's site is gone (as is the download location), so my next best recommendation is
Seashore, which is also a basic paint program.
A nice program to beatify your images is
ImageWell. This isn't so much a paint program (although you can paint with it), but it has some features which might be useful (such as integration with .Mac). For a video introduction on ImageWell, see
ScreenCasts Online episode 21.
All programs I mentioned above are freeware.
Have fun drawing
