Ok, lets try. Open photoshop, go "file > jump to > imageready".
Firstly, Open your sig picture, and go "Image > Image Size" and see how many pixels wide and high it is. Remember them or write them down or something.
Now open the animated gif file you have, then at the bottom you will see the Animation thing. If you press play it should play the gif you opened. Go on "Image > Canvas Size" and change the width and height to the size of the sig (assuming the sig is bigger than the animation that will be over the top) It will make a blank (white and grey checkered area = nothing there) area. Make sure you are back at frame one, and make a new layer (to do this, look at the layers palette on the right (by default) (if its not there go "window > layers) and at the bottom of it you will see an f, a grey square with a white circle, a folder, a piece of paper with the corner folded, and a a bin. Click the piece of paper and then drag this new layer down the stack, so its below "layer 1" (ie its at the bottom of the stack). Now go back on the open the background sig you have (i assume that doesnt move/isnt animated) and press ctrl-A (to select it). Go back on the gif file and make sure you are on the blank new layer, and ctrl-v. Press play and you *should* have what you want
Now, "file > save optimised as" and save it with the name you want (obviously
) If any of that is unclear ask, ill help best i can! If anyone knows a better/quicker way, say