Everyone is talking about installing GIMP,an image editor in the X-Windows environment [Editor - Which requires a long, complicated install of first X-Windows then the Gimp], but another quite good application is TIFFany 3, from caffeinesoft. Written in Cocoa. They are also building a server for imageprocessing in batchmode (T3-server) and some other small apps.
You can find a demo at;
http://www.caffeinesoft.com/
Mac OS X Hints
http://hints.macworld.com/article.php?story=2001042504323144