Is it illegal to take pictures of money?

You can't scan money. If you try to print a picture of "nice flat cash," newer versions of Photoshop won't let you do it, either. If you take a picture of money that is not something that could be printed and ped as legitimate currency, there is no problem. I doubt that your "artistic" photo of money could be printed and used as a counterfeit bill, so just go ahead and do it.