Many small gif files take on the back ground color of the forum. They have no fill color and are transparent, yet they are still the animation. I've seem them on many of them, but how do you do this in flash? Is it the eraser tool?

uh imageready i would suggest, it is easy to make animations w/ that

imageready is a program that is partners with photoshop, so when you finish your image you can take the psd file into imageready and make animations and such

it is also good for slicing and coding websites but thats another story.