Great concept
I think it's excellent that you've made a game that can challenge the brain, and educate people about the classics at the same time. A very interesting concept, and it forces you to analyze each piece in order to find the subject of the question.
However, I wouldn't call most of the questions riddles. Many of the problems were simply just: "Find this" or "Click That". The quality of the paintings was also a little trying for finding certain things, but then again, there's only so much you can really do when you're digitally transcribing 500 year old paintings into flash. I did enjoy it, and I would encourage you to create more games with an emphasis on education.