Is that true? If Zillow doesn't own the copyright, presumably they have some kind of license. Perhaps it has clauses like "we, Zillow, won't allow/abet/facilitate the use of these images on sites other than zillow.com, and/or for purposes other than advertising houses for sale".
Now, when you take these images and use them on another website for another purpose, who has standing to sue, and for what?
To put it another way, when you visit zillow.com, under what license are you downloading their images to render on your device? Surely under an implied contract you have with Zillow. Does the fact that they don't own the copyright prevent them from enforcing any limitations on your usage at all?
"Except as expressly stated herein and without limitation, you agree that you will not, nor will you permit or encourage any third party to, reproduce, publicly display, or otherwise make accessible on or through any other Web site, application, or service any reviews, ratings, and/or profile information about real estate, lending, or other professionals, underlying images of or information about real estate listings, or other data or content available through the Services."
So yes, McMansion Hell could be violating their terms of use.