Just to expand on the above. This rules also applies to pictures made with programs similar to Heromachine such as Hero-O-Matic, as well as hand drawn artwork of your own creation.
Here are some words by Jeff on the matter taken from a Blog post on the Main Heromachine.com page:
Because this is (or at least, was) a business, I am in a much more tenuous legal position than you are as individuals. And yet, hereâ€™s a case where a guy did some cool Wolverine sketches and had to hire a lawyer to keep Marvel from retroactively taking them for their own.
Make versions of copyrighted characters at your own risk, folks. But if you do, you canâ€™t post them here.