Ironically, I think the site that has killed the market for commercial photos is Yahoo's other free photo hosting service: Tumblr. Image copyright is now practically a dead letter if you're an amateur.
Why would an amateur care about copyright? The whole point of copyright is to grant a monopoly to a work's creator in order for him to make money. If he's trying to make money, he's not an amateur (viz., someone doing it out of love — hence the word).
An amateur certainly wouldn't care about the market for commercial photos!
Whereas tumblr is built on - literally made out of - the reproduction of images without the permission of the copyright holder.
There's no RIAA or MPAA for photographs that has the clout to force platforms to redesign their systems to pro-actively check for copyright infringement.
(I'm trying to scrupulously avoid saying which side is right here, just describing the situation)