This Frank Frazetta painting was used for more than simple inspiration. As you can see in the graphic above, the artist cut and pasted Frazetta’s work into his own digital file, and ran it through some filters in hopes of changing it far enough beyond the original.
The rule of thumb is to be safe and not use copyrighted work in your own art unless you have permission to use or a citation of the source. But if you are going to use copyrighted work in your design, it needs to be changed so far that no one could possibly see the origin of it. The designer did attempt to change a lot, but in this example above it is still far too easily recognizable.