Seeing that Ubuntu is moving to LightDM with 11.10 and above, I'm assuming that Linux Mint will as well. I came across a link a few days ago (http://www.omgubuntu.co.uk/2011/09/tool ... ntu-11-10/) about about customizing the background wallpaper and I think its good that the devs know about this (if they don't already) and maybe integrate some customization options in one of those Mint specific config applications.
Maybe you can give three options (user can only pick one) for LightDM:
1. Option to use the same background as your desktop background
2. Option to manually select an image from the available system backgrounds or browse hard drive
3. Option to use a solid color for the background
EDIT: It seems like someone has put together a simple GUI: http://www.webupd8.org/2011/10/simple-l ... ghtdm.html