AppleScript allows for up to 7 custom login backgrounds.
LogPicker allows you to choose up to 7 customized login backgrounds for Mac OS X. But be careful, this is an AppleScript with some assembly required. You can't just unstuff the download and expect it to work: you have to get your hands a little dirty with Apple's Script Editor. Don't worry though, all the instructions are outlined in the "Read Me" file and in the script itself.