For those of you who would want to be able to change the login background, and are running LMDE Sid+MATE.....here ya go;
1.) Go into; /usr/share/gdm/greeter-settings
2.) Then click on xy_desktop-base.gschema.override.....this will give you the path where the login backgrounds are kept.
3.) That would be: /usr/share/images/desktop-base/login-background.png or .svg whichever the case may be.
You can change this by openning the above folder by "Open as Administrator" on the drop down through right clicking on the folder.
You'll also have to copy & paste any replacement pics in said folder as "Administrator", this gives you superuser privilages.
4.) While inside desktop-base folder and changing things around, make sure you rename your new pic "login-background.png" and renaming the old background first before the new one. After that, just close everything, reboot and whala, your new login background.
Standard LMDE without running experimental or sid or testing, etc. .....just plain LMDE Stable, your backgrounds are in: /user/share/backgrounds/linuxmint-debian. There are 4 folders in backgrounds, but the folder you want to do the switching in, is linuxmint-debian. The instruction for changing the pic is the same as above, using "Open as Administrator" while right clicking linuxmint-debian folder....you have to have administrative permissions to do the changing
GDM is similar to GDM3 & the backgrounds are held in the same folder....instructions are the same.....the most important issue here is, please be very careful while doing this and always be aware of what you're doing with no distractions.....I hope this helps anyone who wants to adventure into tweaking their LinuxMint system.....cheers
Dell Latitude D810/2.26Ghz Intel/2GB Crucial/250GB WD/nVidia Quadro FX 1400 256MB/Intel Pro Wireless 2200BG
Win7 Bus 32 bit/LMDE-Sid-MATE 32/64 Tracking Sid