This question contains spoilers for David Mitchell's Slade House.
In the first three chapters, the narrator will at some point ascend the stairs in Slade House and walk past a bunch of portraits. These seem to be very accurate portraits of the previous victims of the Grayer twins as they looked at the time of their death, in chronological order. The last portrait in the line depicts the narrator themselves, but without eyes (whereas the other portraits all seem to have eyes). As far as I remember, the portraits themselves are never mentioned explicitly by either the twins or Marinus in the book (the only people from whom we could expect an explanation).
It's been a while since I read The Bone Clocks, but I seem to remember that the Blind Cathar is also a painting without eyes. This doesn't seem like a coincidence, but I'm not sure what to make of it.
So what is the significance of the portraits? Is it just the twins showing off? Is it some sort of manifestation of the previous victims that they can't control, much like the "ghosts" of the hairdresser and of Gordon that show up? They don't seem to be a necessary component of the orison, since Freya doesn't see any portraits in The Fox and Hounds. Why don't the portraits have eyes until the victim is killed?
I suspect that a complete answer might have to draw upon the mythology established in The Bone Clocks, unless I'm missing something obvious in Slade House.