People tend to think single photos are worth far more than they really are. They hear about the one that went for $10K, or the one that earns $100/month from stock. Then they conclude that every half-decent photo is worth that.
I think I've posted exactly one photo on POTN that somebody else might want to print. As others have said, it's a low-res posting. Even if I did sell it, I doubt I'd get enough to stress about.
(I have had three shots from my general Web site published. One earned me money; the other two I gave away free because they were good causes. In all three cases, the people who used them contacted me for permission and high-resolution copies. Maybe there are more that I don't know about; again, I'm not stressing. I doubt that anybody has made thousands off me.)
(As a side comment, I'm responsible some open-source software that people have contacted me to ask if they could use it. The first time I think I asked $10K; I figured that any company could afford that. No sale. On subsequent contacts I kept dropping the price, never getting to agreement. Now that I know the true value, I just give it away.)