GrafX2 [1] is a very nice editor, if limited palette is acceptable (you can forget about editing truecolor photos). Interface is taken from Deluxe Paint. Code is C with dependencies on SDL and Lua (can be compiled without Lua). There are binaries for Atari available, so it is a lightweight editor compared to gimp.
[1] http://code.google.com/p/grafx2/ -- Paul Onyschuk
