I have added a few books today and while I was trying to figure out who made the cover to Dark Universe (Daniel F. Galouye) I found this nice album on Flickr with some nice classic sci fi covers collected by Eric Carl.
And if that wasn´t enough for I suggest you make a Google image search of books written by Edmund Cooper. Christ they look awesome. As an example check this one out by John Berkey.
Keep on star roving.