Visual artists such as photographers and illustrators find themselves in a position similar to the owners of a car fleet that mostly has no numberplates. Most of the photos and illustrations washing around on the interwebs are "orphan works", even very famous ones, as they have no attribution or identification attached. And although the law is quite clear about usage, which obliges the potential user of a photo to discover the owner and obtain their permission, with no contact information (or out-of-date contact information) many users of the art just don't bother.
Naturally there are powerful vested interests who want to make it even easier to acquire this property, minimising the expense of finding out who the property owner is – and paying them. Especially paying them. These forces range from multinationals such as News International and the BBC, Google and Facebook, to Big Culture institutions on a mission-creep. (You've never heard of a bureaucracy that abolishes itself – and you won't find a library or museum that doesn't want to turn itself into an uber-publisher or New Age licensing authority).
Okay, maybe the "topic" and byline submitted was over the top, but if you are a photo centric site, you need to read this overview of WHY YOUR WORK will be orphaned/used/ripped. And there's not much you can do about it short of expending REAL MONEY with lawyers.