Find somewhere a theme for gdm... download it...
I think it is in tar format... And it should stay that way if you want to use it as a theme...
I modified my like this...
Untare the theme i downloaded and (i guess u are trying to change the picture) go to the directory you untared it in...
Look for picture and look in what format it is...
Take another picture in gimp (the one you whant as the theme) and save it under the same name nad extension as the one in theme directory u unpacked was...
So if you download Angle theme then it would be angel.png...
Tar it u p again..
Run gdmsetup and install the theme...
WARNING!
The theme name will be the same as the original...
(I don't know and don't care how to modify it)
This is the firs i found on quick search...
http://www.desktop-linux.net/gdm.htm
If you want mail me i got a few others if you like sculls or vampires...
