I remember playing at a science fiction cover design site. I can’t remember the name, but it let you formulate your own cover from stock picture elements, and post the titles and subtitles.
Would it let you actually use the image on an actual without paying royalties?
selfpubbookcovers.com is not free but I guess $69 ain’t bad if this is something you make money at.
This site looks like fun, but I dunno about copyrights and stuff
http://thrilling-tales.webomator.com/derange-o-lab/pulp-o-mizer/pulp-o-mizer.html
really really old pictures might have lost their copyrights...