I came across this a while ago and thought it would be good to share. A fellow named Levar, who runs the blog Vintage Paperbacks, has a ton of old sci-fi and pulp covers posted on flickr. His collection of covers by sci-fi artist Richard M. Powers is particularly large and the covers are amazing.
Richard M. Powers, who died in 1996 at age 75, was an enormously influential artist in the realm of sci-fi. Influenced himself by mainstream surrealists like Picaso, his covers had a distinctive visual style that was often more intriguing than the stories that illustrated. I can sit and look at them for hours. Sometime I do.