Thanks. The interview says that Casaro received reference photographs of the Almeria settings as they were filmed. So yeah I guess he made at least some of the posters before the movie was finished. It would explain why Conan wears a loincloth, a scabbard and a helmet that is not shown in the final movie. (Btw that helmet is not as "original" as the guy claimed, it comes from a Frazetta painting called "A fighting man of Mars", it's the helmet of the princess
The scabbard is also a Frazetta one.