Flash is the easiest way I know (just because that's what I started with), but there are many...many...programs that will allow you to stack images into an animated GIF. Do you have all the images that you want stacked?
...and if you have Flash, it's pretty easy. Just set the size of the window to the dimensions you want, import the picture, copy it on each layer and modify it however you want, then export as a .GIF file.
Otherwise, you need one of the other billion or so programs to do it. I'm not sure which ones are better, but there are loads of them on Google's results...just search for free GIF maker, or something along those lines!
If I can help in any other way, LMK!