A very well done site! The ability to jump in and make a card and get a shortened link without signing up and harvesting email is one of the real attractions, the other is the flickr integration, its really slick.

A possible problem is you may be violating the license to some of these images. The first few I checked out where all creative commons, but this one does not allow derivative works, but it shows up in the results: http://www.flickr.com/photos/24879866@N05/3488893875/

(I may be misinterpreting the license)

One minor thing, where it displays the images from flickr, I would make the "use" button more apparent, it doesn't look like a button so it is not clear that you can click on it.

Otherwise its a very nice site, how long did it take you to build? Technical details?