Atlantis by Frank Frazetta

Wow.  I will never forget the consternation that little square painting on the paperback caused in my circle of geeky friends, the geekest of which actually read the book (and its predecessor).  Personally, I dig the first image the most, Betty Ballantine's choice of how it should be presented (straight from the World Beater poster, according to her, but I remember the poster being pale in comparison)