I want to change my login screen background (and keep the dots). But I upgraded to cinnamon 2.0 a day ago and then to 13.10 earlier today. I recently became aware of the issue with cinnamon 2.0 and unity on 13.10. This normally would not matter to me since I use cinnamon instead but the only way I know of to change the login screen background is to change unity background. Also all of my backgrounds are jpgs not pngs (in case anyone was gonna say the replace /usr/share/backgrounds/ubuntu-warty.png) and for some reason gimp has issues so converting them is not currently possible for me.
Hope Ubuntu-Tweak will solve your issue
Hope it will help you to resolve your issue.. Reply if something goes wrong..