On how to do this: it's easy! You preferably need photoshop w/ ImageReady installed. There are other gif-animation programmes out there, but photoshop integrates the animation function with the coolest photo edit programme around. Then you need a lot of time... I can tell you: the idea is the EASY part, the execution can be really cumbersome.
Like with the Milt cover: I had to extract each of the little balls and animate it seperately. Pity you hardly notice that in the resulting gif. The reconstruction of the background image can also be a drag (like w/ the Dexter, the Young, and the Milt).
You simply make a lot of layers, each with a frame or a part of it. ImageReady lets you pick the layers you want to display, for how long, and in what order. I only really learned how to do this recently and this exercise is definitely good to improve one's skills