I often see professional and amateur artists posting drawings of characters they don't own. Some sell "con sketches" of Marvel's Hulk, others create erotic fanart to Assassin's Creed.
Who is the copyright holder of this new work? Do I have to ask the copyright holder of the characters, or the artist of the derivative work, if I want to use such an image?
Who can sue me, if I create a derivative work off a derivative work? The creator of the fanart? The copyright holder of the characters? Or both?