Well if you want a moving picture in your signature then you must save your animated file as a GIF...you can make it a gif in Photoshop or perhaps GIMP(from what I heard)
Once you have made your animated file into a gif, then take it to the mentioned websites from above or perhaps going to Photobucket(another pictures sharing website)
Once you uploaded your picture from there, they will provide you with codes on the right side...one of the codes will be IMG..that's the one you will need. Copy and paste it in your signature and there you go.
NOTE: there is a size restriction when putting a signature in, it can't extend past a certain amount