Is there a way to change the background on the login page?

Yep. Inside HD/Library/Desktop Pictures you'll find a file called Aqua Blue.jpg change this to Aqua Blue1.jpg. Now, get the photo you want for the login screen and change its name to Aqua Blue.jpg. Place it into the folder and you're good to go!

Another way was posted at macgeekery earlier this year:<pre style="overflow:auto; padding: 5px; width: 500px ; font-size: 10px; border:1">sudo defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture -string "/path/to/image.jpg"</pre>The command should be entered using "/Applications" > "Utilities" > "Terminal.app" while logged in to an "admin" account, and substituting a proper path to an image file, in the appropriate place.

The "renaming" way is probably easier for people unfamiliar with the command line.