Nice idea. I'd recommend implementing a demo image on the site. Not everybody will go through the trouble of uploading their own file to test out the project.
Yeah, once upon a time I had an idea of having a drop-down menu which would have four or five sample images you could select from to instantly load them into place. I can't remember why I didn't end up implementing that. But unless a new client drops out of the sky in the next few days, I'm going to have some spare time later this month, so perhaps I'll take your comment as motivation to hack that into place. :)