I like the idea also, although it's a bit pricy at $16.99 for twenty $.37 stamps. (Not sure if their price goes up in Jan. [Jan. 8] when our stamps go up to $.39 ?) When I first heard about it, I started looking through my images for ones that would work as a stamp -- image area is 1.1in X 1.1in, while the stamp itself is 1.9in X 1.4in. The upload process lets you preview the image as a stamp and make very simple edits (zoom, rotate), but you'll probably want to edit a bit in Photoshop/
PSP/etc.