Messed up my graphical login HELP!

[Andrew Konosky <TerranAce007@xxxxxxxxxxx>]

I was installing some new graphical login screens, but on of them must 
have been a bad download because now when I boot up, the login screen 
gives me an error saying that the PNG image is bad. I press OK, but the 
error doesn't go away and I can't logon graphically. I can log onto my 
system via the command line, but I don't know what the commands are to 
do anything. How do I reset the login screen to default via the command 
line, or how to I launch Gnome or KDE from command line so I change go 
back and change to my old login screen?

Thanks,
Andrew