It's common courtesy and standard internet protocol that when you post, via a hotlink, to someone's photo, and are using their bandwidth, that you provide a link back to that person's source page which is hosting the image. Also, if you save an image and upload it here, that you credit the photo to its original source. This is practically an epidemic here and is no different than posting an article without a link to its source or who wrote it. It's basically photo plagiarism.
My suggestion is that if this isn't already a rule of conduct here, it should be, and should be made clear in some way, and of course, enforced.