If you're in need of some high-quality artwork for a prototype or even a full-scale production, check out the Digital Comic Museum. There are over 15,000 golden age comics released to the public domain, free of charge, complete with high-res scans of covers, interiors, even the ads! If you dig deep enough, you'll find work by Ditko, Schuster, and others. So many genres, too: Westerns, Romances, Space Adventures. Literally, a comic called Space Adventures. I'm definitely going to tap it for some scifi game in the future.