I heard they use a company (marketplace) to order photographers to shoot their listings.
Airbnb actually developed an internal competency to manage their photography process. Turned out it was/is core to their site and found they needed to own the process. So they have a team at Airbnb that manages relationships with thousands of photographers (who are independent contractors).
While at Zaarly, we considered a number of options for our photography on the site but ultimately decided to go a similar route as Airbnb because the quality, look/feel and process mattered greatly to the individuals we featured on our site and it mattered to the general design and feel we wanted to provide.
Good photography matters. If you want to do it well, don't underinvest or try and cut corners. You can quickly tell the difference between professional shots and stock photography.