Not quite sure where you're up to but it'll be one of two things.
If you're in Aeval's house, then you click on the picture and a tune will play. Then you click on the piano, turn the crank and the tune will play again showing you the notes on the piano. Duplicate the tune and the picture will resolve into an image. If you get the notes wrong, just turn the crank again to reset it and try again.
Do this for each picture and when all are resolved, the door will open.
If you're at the mansion, then click on the door to the top left to continue.
Cheers