Go find a 'open shade' wall of a building and drive a nail in it. On a dry, not too bright wall you can take perfect pictures-of-pictures or 'reproductions' and forget about the flash altogether. You should stick to the tripod though, and I would also suggest a remote triger and mirror lock-up if an SLR.

Many years ago I had to take photographs for an art publication and went through all the previous suggestions until a friend of mine (now living in Orange County CA) told me to turn the paintings upside down, it has to do with the way the brush strokes are made and lo and behold end of problem. Then try to light it from 45 degrees.