I need to know if there is a web-based app out there that will allow you to
upload an image and have it overlap a background say for promotional items.

So i'd have a viewer in the middle. Custom uploaded images along with
sample images on the left . The promotional items, e.g., mugs, shirts, caps,
etc., on the right.

I would like a customer to be able to select a promo on the right, it loads
in the viewer. Then they would select the image they want to see on the
promo and then it would be loaded in the viewer achieving the overlapped
image on the promo item.

Is there any 3rd party web app or service that will handle this for me or
will I have to create this myself? If i do need to create it myself, how
would i go about doing this. Is it as simple as 2 div tags that overlap with
one set to transparent? It seems to me that the hard part would be removing
whitespace around images people upload.

Any help would be much appreciated,
Scott