OK I fooled with it.
If you don't like the themes they are real hard to edit.
Go to /usr/share/mdm/themes/linuxmint in a root browser. I use Xfce and have a "custom action" that will "Open With MInty Green Authority" from the right click menu any directory.
Open the above theme as root. Pullup the "theme.xml" file in your favorite text editor and you will see that you can have a background color.
That is defined by the numeric color stuff (don't know what it is properly called). It is easy to get the code for whatever color you want by fooling around in Gimp for the background or foreground of an image. Get the color you want and the number will be copy/paste-able from there to the file.
This will only come up if there is a problem with the background.png setting which is next in the script. If you want a solid color then comment that whole image section out. If you want an image get one you like, remembering that the log in box will be plumb in the middle of it (although you could change that too). I used a simple image that i made years ago that just looks like green burlap. I used Gimp to make it the same resolution as my screen size.
Created a new folder in that /linuxmint folder "aholding". I like a holding folder in config files that I edit to hold the defaults that are changed and I like it at the top of the list, thus aholding.
Put the background.png image in aholding (or whatever you call it). Put your image (.png to be safe although I suspect a .jpg would work) in the theme and change its name to "background.png". You could also leave the name what ever it is and edit the theme.xml to read the name you have on your image. You would need to edit the .png to .jpg in any case if that is what you are using.
Save the theme.xml file close things down and reboot. You should now be using that image.
I did not do anything more in the theme.xml file. You could, however, simply edit all the colors and sizes in there and then save it to folder by that name of your new theme.
Basically just take the entire original theme, modify it, and save it as another theme.
These things look pretty easy to make. You may become famous for your themes if you enjoy it enough to fool with.
Remember that if you simply copy the theme to someplace safe you can do anything you want and easily revert to the saved copy.
I figure that couple of minutes I spent on it was plenty. The login screen is only up a few seconds anyway. I do like to change the image.
Thank you for posting this thread as I probably would have suffered on with the default forever as this is not my production OS. Looks a lot better when logging in now.
"Screen Shot" is made up in Gimp without the little doo dads added to the login box but will give an idea what it looks like anyway.
- background.png (493.79 KiB) Viewed 4068 times
Dell XPS 420 Core2 Quad Q 6600, audigy5.1, Radeon HD 6450 - currently 4 320Gb HDD, Debian Squeeze for secure use, Debian testing for daily use, Debian Sid for fun.