Sorry for the delay, Jimmy- sometimes these things are hard to find.
After going around and talking to the suspects (you need to talk to everyone) you should be narrowed down to two people- Orgnolf the castle porter and Chanel the mage. Head into the private quarters and look for the door leading to the West Tower. You should find a trap door behind some boxes to your left here- you'll find a painting and some painting supplies. Head to Chanel's room and you should find some more painting supplies in the Lectern, giving some evidence she was the one responsible. Head down to the Dining Room and check the rug under the table- you'll find some paint stains to examine.
With evidence in hand, time to confront Chanel, who will admit to the crime. If you tell the countess it was her you will get a hefty fee in money and gems. However, if you tell her neither of them were the culprit you still get some gems for your troubles. If doing this, go back to Chanel and tell her what you did- she will paint you a portrait. It will be ready in 3 weeks.