"There was an error loading the theme Default"

Hello,

I'm running a redhat 4.0AS server. After one of the
updates I ran a while back, I get an error when trying to log into the
machine:

"There was an error loading the theme Default"

"Couldn't recognize the image file format for file
'/usr/share/gdm/themes/Default/background.png'"

If I vnc into the machine, all the images are blank with a red x in the
center of them.

Any ideas on what the problem may be?  It's almost as if it no longer
will display png files.

- jody