I would go to the gallery concerned and, as someone has already said, try to sell them prints of the painting while at the same time giving strong hints that the artist may be breaking copywrite laws and that the gallery is aiding and abetting said breakage.
But before you do that, have you actually spoken to the artist? He/she may not have thought what they were doing was at all underhand and the painting is up for sale in all innocence.
As another responder has said, I'd be quite happy for someone to use some of my pictures as I probably wouldn't be making any money from them myself, but I would absolutely INSIST that I got a credit for the work.
I have a long story relevant to this subject but it would be a digression here, suffice to say, resending my work to the illicit user with copywrite symbols all over the bottom of them resulted in said photographs being removed from their website rather quickly.