I use photoshop imageready for the animations I have on furtopia. You
build the animation with layers in photoshop, then port them to the
animation software in Imageready. Gives one a lot of options for timing
and frames.
Nice thing is it has the option to compress the animation down to a
reasonable file size, and display size too.
I imagine Gimp can do it pretty much the same way.
All that's left is the hours of work.