In reply to gunbo:
If they are in a public place, they can take pictures of anything they wish. If it's private property and there are signs saying no photography then that's different.
If they are a stock photographer, they wouldn't be able to upload the image to any Micro or Royalty Free agency as they would be asked for a property release before they could sell it.
It could be uploaded to a Rights Managed agency but again, without a property release it could not be used commercially. It could however be sold as an editorial image, for example, used in a newspaper or magazine to illustrate an article or news event.
As others have mentioned, it is very unlikely they would get a good enough image to use as a print, it would need to be in a studio to do this so you could control the lighting, shadows and perspective etc.
If the image they took was of the painting and stall / surroundings, then I would say it maybe an editorial stock photographer. Still not much you can do about it if it is a public place though and shouting at people to stop talking photos may not be the best PR.