I used to have GIMP a long time ago and used it to make .gifs, but I don't currently have it. Basically, you need to paste/draw the sig you want to edit on the first layer (make sure to copy it once it's set). Then you select an option (I think it's add frame or layer) and paste the same picture in that layer, but now add the tweak to it. Then copy this layer and follow the previous steps in that cycle.
I may download GIMP again and figure it out for you.