I may have found the problem, at least on my system. All the themes that do NOT garble the login screen have a "background=true" statement in the XML file. The SUSE does not. So, I went to
/usr/share/kde4/apps/kdm/themes
Opened the SUSE directory, then opened suse.xml.
In the second line after the <greeter> statement, I added
background="true" after the ID, so it looks like:
<item type="pixmap" id="background" background="true">
That fixed it, so that there are no garbled texture artifacts, and the background fills both monitors on my setup.
So far, this has been working after several logouts and reboots.
I'm not a programmer, I don't write code, so I don't know if this is advisable, but it's working for me.
Cheers!
