> But has anyone done that?
Good examples of paid-for TwitPic photos include pictures in the wake of the Haiti earthquake, or the pictures taken of the Boeing which emergency-landed in the Hudson.
> Whey would they pay, given that it's available anyway?
Because they have to—the user only relinquished part of her rights on the picture, namely distribution on the Twitter/TwitPic medium, not her entire rights. If you think that were the TOS to change, the users would still upload pictures, you're so very wrong—see the TwitPic debacle recently and how MobyPicture gained a few k users because they state in their TOS that they can't sell their users' pictures without prior agreement.
You seem to think that your analogy is good, but I don't think it is. Were a magazine to publish SO answers in a Q&A section in a paid magazine, I think SO would be subject to licensing fees. In that case, the relationship between SO and you is governed by their TOS. Check them out, you'll have your answer.