LibraryThing has many features you can do neat things with, e.g. the "All your covers" display. This will display small photos of all your book covers on one page - by using Firefox File/Save Page As/File Type/ “Web Page, Complete,” you can save them a folder on your hard-disk, and easily weed out extraneous graphics. You can then import the pictures into Picasa and make "suitable for framing" collages, like this one, and turn them into cards or posters.
Another idea is to make a photomosaic of the pictures using the freeware program AndreaMosaic. We recently put together a collection of "hyphenated-American" fiction to present to a local library. Prompted by Horace Engdahl's sage remarks about the parochial, "insular" nature of U.S. literature, I decided to arrange these works by U.S. writers from all over the world in a photomosaic of the Statue of Liberty (looks best if you squint). Cheezy, certainly, but let Horace put that in his pipe and smoke it! Insular, eh? The whole procedure is explained with greater clarity at David Edelman's blog - thanks David, and thanks LibraryThing!