Probably the avenue for monetization would be access to the back catalogues. Which in turn eliminates the free part of free stock photography.
While this may seem ludicrous (eliminating the appeal of your free stock photos over paid ones) this is common practice after a series of emails looking to elaborate on this point with a few of the services.
The whole thing seems a bit flimsy to me though: (1) Don't provide free stock photos at the time that people want them. (2) Later doe provide stock photos at the time that people want them, but no longer for free. (3) Profit!
Interestingly, this business model incentivizes subscribing to every such website's mailing list you see on the off chance you might need the photos later.
While this may seem ludicrous (eliminating the appeal of your free stock photos over paid ones) this is common practice after a series of emails looking to elaborate on this point with a few of the services.