Short answer? Never.
Long answer? As a self publisher / writer, you are supposed to play by the rules. You wouldn't want someone using your work without authorization, so why should you use someone else's art work without authorization? Without the right? Especially if you want to be considered "legit".
We are not talking printing something out for your own personal use. We are talking something sold to your customer base (BTW, even something given away for free is considered a sale by law - ask many a pot smoker who passed along a doobie amongst friends, only to have each and every one charged with "sale")
Not going to name names, but I will show the artwork in question, published by DC Comics in 2012.
Maybe the publisher doesn't consider themselves legit. I don't know. I just don't understand the mindset behind appropriating artwork that isn't yours and then signing the cover on top of that.