By law, any photograph shot by you that you post on the internet is owned by you, the creator of the image. You own the copyright to that image. If someone horks your image off the internet without your written permission and creates a painting, it's in violation of the copyright laws. They have broken the law. You have a legal right to send a bill, collect payment for use and/or demand the image (in this case, a painting) created or proof of destruction of the image (and all copies) they created. If 10 copies (giclees or prints) have been made, it becomes a felony including a possible damages award of $2500 regardless if any profit has been made by the violator.
I think it hurts the industry that so many people are "flattered" to be ripped off. It diminishes the credibility of your photography and the photography industry in general.
Though many people out there in cyber-land aren't aware of the copyright laws, it's up to you, the creator and owner of the image, to educate them. Please do a search to learn more about the copyright laws. Here is a link of links that might get you started: http://www.photosecrets.com/links.law.html
No one is saying not to share. .. but I'm encouraging you to share the correct way, by giving credibility to the laws and to your imagery. (ie: issue a licence for use) If your image is good enough that someone wants to steal it to make a profit from it, then it's up to you to make them aware of (either) their ignorance of copyright laws or (if they don't care) their liability of the theft. (not referring to the OP .. because the artist did ask, which is commendable)
When someone does ask your permission, (that's wonderful when they ASK!), then it's up to you to issue a licence for use or not.