It's always safe to contact the artist first for the rights, and they may give you a photo they like. However, the rights to some pieces executed prior to 1976 (last copyright law revision) may be controlled by the person that owns it, so ask them as well. And some older pieces may be in the public domain by now so you can take your own photo and use it. Finally, if you want to use a particular photo of the work, the photographer probably owns the rights to it so you'd have to ask them, or purchase the rights from a stock agency.