I am yet to flag anything yet, I think. If I see stuff that I feel is inappropriate, I don't upvote it.
And yeah, I know the value of stock photos, during my time in the TV broadcasting team, finding some stock royalty free photos was part of my job. A very minor part, but nevertheless. :-)
So far I had no reason to add stock photos to my writing, and I am going to keep it that way. I've used a book cover, when I was writing about a book, and a logo of Slashdot and Bitshares, when talking about them. But that is it. :-)