Blogger Kevin Huxford has an email exchange with Blizzard posing a hypothetical:
What is Blizzard’s policy on screen shots being used to create a webcomic and then selling advertisement on the webcomic site? I’m worried that compensation for ads on the same page as the webcomic would constitute commercial use.
Not shockingly, Blizzard says this is against their current policies. Now have I heard of Blizzard going after people for this? No, not really. But anytime you use copyrighted material in your work, under the current law of the land, you run a huge risk because the copyright holder holds a lot of cards. Yes fair use can be a defense to what would otherwise be copyright infringement ,but it is a case by case analysis and you really need to know how to do that analysis (or have someone do it for you) to know whether you can make a plausible claim to that or not.