Frank R. Paul

Austrian born Frank R. Paul was the first and best true science-fiction illustrator.
Here is his official website, with many beautiful pictures of his covers:

Official Frank R. Paul Website

Here is a short bio from the site:

The Encyclopedia of Science Fiction has this to say about Frank R. Paul: "FRP is the best candidate for 'Father of Modern SF illustration', at least in the form it took in the pulp magazines. He received much of his education in Vienna, and studied also in Paris and New York. Trained as an architect, he was discovered by [editor] Hugo Gernsback in 1914 while working for a rural newspaper. Their names have been virtually inseparable ever since the days of Electrical Experimenter. ... For #1 of Amazing Stories in Apr 1926 FRP not only painted the cover illustration but did all the interior black-and-white artwork as well, and continued to do [all the covers and many of the interiors] until Gernsback lost control of the magazine in 1929. When Gernsback started publishing again later that year, FRP was once more his primary illustrator, on Science Wonder Stories,  Air Wonder Stories and then Wonder Stories; indeed, his association with Gernsback lasted until the short-lived Science Fiction Plus in 1953 [and beyond, if you consider Gernsback's Forecast magazines - FW]; he painted more than 150 covers for Gernsback in all [closer to 190 if you count Science & Mechanics and Forecast - FW]. He worked elsewhere, too, with a further 28 front covers for various non-Gernsback SF magazines, including all 12 for Charles D. Hornig's Science Fiction, and also a series of full colour back-cover paintings for the Ziff-Davis Amazing Stories and Fantastic Adventures (1939-1946).  He also did all the illustration [correction, all the covers and some comix - FW] for Superworld Comics, a Gernsback experiment of 1939.