You can customize the Lion Login screen background image. This hint shows where to find it.
After doing some research (thanks to 'EasyFind' that looks inside packages), I found the original file that is used for the login texture background image in Lion.
The file is called NSTexturedFullScreenBackgroundColor.png, and the path to get to it is:
/System/Library/Frameworks/AppKit.framework/Versions/C/Resources.
It is a 256 × 256 PNG image, so if you want to try replacing it that's what you should use. I uploaded a screenshot of the path to the file here.
[crarko adds: Have a bit of caution when changing a file like this used by the system. Mostly make sure you have a good backup around, and don't be too surprised if some software update reverts back to the original image.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110704005342103