Unfortunately, there are few free Mac image editors. One free editor commonly included with Linux distributions is The GIMP
(strange name, I know.)
Several groups have ported the GIMP to Mac OS X, and a free version is available from Sourceforge
. You need to also install X11 for Mac OS X
(available on the Mac OS X disc under "Optional Installs").
See this tutorial
to get started with animations in the GIMP.