It's based on Frank Frazetta's painting of Conan called "Barbarian." The piece has been used a couple of times since its debut but from memory without digging in to find out I think he just did this one as a standalone painting.

It wasn't a standalone piece. It was the first of a series of covers he did for Conan books in the 60s. This particular one was for Conan the Adventurer. And since he owned the rights to the artwork he was able use it for prints,posters,calendars,etc as well as license it out for others to use.

Already answered :)

Gotcha, thanks for filling in some gaps there. I remember seeing it as a poster on its own and then later as an ad for the '82 movie but you're right it came from the book first.