In Lion, the background used for the login screen is located at
/System/Library/Frameworks/AppKit.framework/Versions/C/Resources/
The easiest way to get to this folder is to choose Go to Folder in the Go menu while in the Finder. Then just copy and paste in the above line and click Go. The image that is used for the background is NSTexturedFullScreenBackgroundColor.png. You can replace this with any other image, and as long as you give it exactly the same name, it will be used for the login screen background. Before you make any changes, make sure you make a copy of the original file somewhere safe so you can go back to it if you want.
You’ll notice that the image is only a small 256x256 pixel square — this is because it is automatically tiled across the screen. If you want to use a similar type of texture, any size image is fine. However, if you would like to use a full size image without tiling, you need to make sure the image dimensions exactly match your screen and the resolution is 72 dpi. Either way, it must also be in PNG format like the original. It’s pretty simple to use Preview to adjust any image to the correct size and format.