First, navigate to:
/Applications/iPhoto.app/Contents/Resources/
and look for a file named 'people-cork.png.' This is the background for Faces in iPhoto. Simply replace this with a new background of your choice. However, it seems like doing this operation requires system authentication. So, what you should do is make a copy of the original image, then delete it from the Resources folder. Then, copy the new image in, authenticating in the process. Remember to keep the old image just in case you wish to revert back later.
A good place to find backgrounds is surprisingly in Apple's Mail Stationary. Simply go to:
/Library/Application Support/Apple/Mail/Stationary
If you hit 'Show Package Contents' on the stationary nested in those folders, you can actually find many great backgrounds. Simply name it 'people-cork' (with a format of PNG).
[crarko adds: As always, when making these kinds of changes, you run the risk of breaking the signing of the application so it won't accept updates in the future. If that happens (with any program) the best bet is to restore the original version from a backup. I don't think it's very clear under what conditions this will occur, so I keep Time Machine backups of my applications nearby.]

