In the system admin general settings in the Login Display card, there is a setting for Login Background. Change that setting to "Custom Image Folder on Server". Point the path to a folder in the MRS directory. You can name this "BackgroundImages", for example. There is no specific size limitation for the image.
Hopefully this helps.
Robert Emmett
Software Developer
SmarterTools Inc.
(877) 357-6278
www.smartertools.com