mariATL
I think what I'd do is this --- go to iStockphoto.com and find a picture that you like of footprints in the sand. Buy it instead of making one (usually $1 or so). then in Photoshop copy the image to a new layer in order to save the original. Then use your clone tool to erase out the prints by copying little bits of the sand right beside the foot to paste over the feet. You now have two images, one without prints, and the original with prints. Export both images separately from photoshop. Import to Flash. Use the 'without footprints' bitmap as your background, then animate a mask to slowly reveal a new layer of the 'with footprints' bitmap on top of the background. Voila, that will make it look like the footprints are being made by a ghost. You can also clip out just one of the prints to use in a MovieClip that could be dragged, clicked on pasted, etc. (not sure if this search will work as a send)

itapirlo
~Here is a tutorial that shows you how to do it in snow, but it's the same principle for sand.