the first frame of the gif is what is used in the final combined image. if you really feel the desire to use animation, just make sure the first frame is friendly within the context of the others. dont want a level going non-blendy now, do we? heh

(eho's piece in 1991 is a well-done example of this. the first frame is great on its own. a little bird that will be lost in time... bonus points for hygrid tie-in!!)