I'm also not really certain that Zillow has the copyright: those are usually property of the RETS/MLS of that specific area, if not the agent taking the pictures.
I came here to ask this question. I've never seen a Zillow ad that wasn't just a copy of the MLS listing (usually a month out-of-date, at that). The same pictures show up on realtor.com, redfin.com, and about 50 other sites.
If you can get the byte-for-byte identical image from a different website, how does Zillow exercise any control here? If she just stops using Zillow, isn't she protected?
Zillow and other Real Estate sites have the same problem: Syncing Real Estate data is hard due to fragmentation (each area has its own MLS with its own rules), clashing data standards, and legal requirements (For example, I believe that Louisiana has a law that requires Realtors use the phrase, "A Licensed Louisiana Realtor" under their name in all marketing materials.) If you're displaying that data, the onus is on you to ensure that you are showing the correct data.
You'd have some MLS'es that required their logo to be on the bottom right, some that required them to be on the bottom left, and 95% of the time, your data feed didn't even contain the logos: you'd have to double check and make sure that you were in compliance (and they did check and would cut you off if you were not in check).
But, to answer your question: There's nothing that prevents her from going straight to the MLS or the listing agent's website to grab her source photos: zillow would have no claim at this point in time.
I'm also not really certain that Zillow has the copyright: those are usually property of the RETS/MLS of that specific area, if not the agent taking the pictures.