I'll have a better look at the skulls next time. I thought I'd tried all the damage types I had available, but I'm guessing that I missed one. I was already getting a bit peeved with the whole design so may have been overhasty

Second time through the house I had Shadowheart with
so that made things much easier.
To be clear, when I say I destroyed
the possessed portrait, I mean that I clicked it on it while holding the torch that I'd got from Mystic Carrion, which caused the portrait to go poof and the spirit of the Lost Love to appear and the quest to update, it's just that she didn't go and find Oskar as the journal said she had.
If that's not what I'm meant to do, I confess I'm really lost!