I've put together a set of covers based on semi-transparent textures. I'm not brilliantly happy with them, but some of them seem to work quite nicely.
The file's about 3Mb (mostly due to one of the four textures compressing much less well than the others, but it's probably the nicest one, so I left it in), and it's available from my Dropbox here: https://dl.dropbox.com/u/39906743/covers.zip
They can be freely used, redistributed, changed, sold (good luck with that), whatever you want.
Here are a few examples of how they blend with the background colour: