Images from google image search are distorted.?

I have a problem with a users iMac. When he does a google image search with any browser (Firefox, Chrome, Opera, Camino, Safari) the image will appear fine as a thumbnail but when he clicks to enlarge it, it becomes distorted in some way. Either half of it will be darker or appear as a negative, sometimes it just looks pix-elated. It only appears to happen with "large" size images intermittently. I've searched about 100 other forums for the same issue but I haven't found anyone with the same kind of problem. This is an issue because the user works in the graphic design department and uses the image search often. Can anyone help?