When I create an animated GIF in GIMP, is there any way to tell it to fade between frames? I just have a 2-frame animation and Google won't accept it as an ad--it's automatically rejected on account of "strobing or flashing" visual effects. They claim to allow animated GIF's, but it just won't accept mine. Also, they restrict animations to 30 seconds, with no infinite loops. Is there a way in GIMP to limit the number of times the animation is done? On the file save dialog, I'm only given the choice between looping through once or infinitely.

You'll have to make the "fade" frames yourself. There might be a way to programatically create those frames, using the two frames you have as keyframes, but GIFs are very simple beasts and don't support any special effects. They just show Frame 1 for Time 1, then Frame 2 for Time 2, et cetera.

The looping has similar constraints. If you want to loop more than once but not infinitely, then you'll have to just repeat the frames you want. If your number of loops is even, then you can repeat the frames half as many times and then loop it once, but that's about it.