Alex Schomburg was an illustrator born in Puerto Rico. He moved to the USA and did hundreds of covers starting from the 30's, for comics, pulps, paperbacks, science-fiction books. I don't really like his super-hero covers during World War II, some of them being racist and too patriotic for my taste. His later science-fiction illustrations are very nice. Curiously, during a very limited time (1947-1948 I think), the guy completely changed his style, and started painting with an airbrush a string of absolutely wonderful comics covers. Then he stopped and never did that again. He signed most of them under his pseudonym "Xela" (Alex in reverse). I love them, they are so kitsch and beautiful, and probably show the most beautiful women legs in the history of comics. Here is a gallery of probably all of these Xela covers. I don't own all of these comics (a single one of them in good condition can sell for several thousands dollars!), but I scoured the web for images, notably from auction sites like eBay and Heritage Comics.
A few links: a short biography of Schomburg, a gallery showing many of his different styles, other galleries here and here and here.