Thank you for doing these. You helped me find another young adult book years ago, and I was so grateful. Unfortunately I can remember almost nothing about this one. It probably was set in the 18th century, and I believe it was in America. It might have been told from the point of view of a daughter or niece of the young woman in the portrait. All I remember is that the artist had captured her glowing and happy as she came in from out of doors. I think she was wearing a cape! There was a man beside her and it turned out the man who had made her so happy had been painted over with the likeness of her current husband. The other man’s portrait was underneath. Does that ring a bell?
I FOUND IT!
This has been tickling my brain ever since it was posted, and I finally remembered that it’s from The Court of the Stone Children by Eleanor Cameron, 1973.
There are three characters involved: Nina who is the modern-day main character, Odile who is the young woman in the painting, and Dominique who was the daughter of Odile’s sweetheart, and who appears as a ghost and helps Nina solve the mystery.
I was just about to come and suggest that book as a long shot! So glad you found it!
I’m not the OP. I just got super excited when I made this connection!