Do you really see Melvyn Peake? When I read the Ghormanghast Trilogy years ago, I never pictured in my mind anything like what I've seen so far for Dishonored.
No? Do you remember how, for instance, the head cook was described in caricatured detail? Almost any character met throughout the course of the story is described in elaborate, exaggerated prose. The way that characters are drawn (with words, not pictures

) reminds me a lot of Dishonored's character design.
But then again the imagination is subjective. Likewise, any of Peake's illustrations I have seen, that I can remember don't make me think of this game, or rather the other way around.
That, I agree with. I don't see any of Peake's illustrations in Dishonored.
One painting that comes to mind is by James Whistler called something like "Old Battersea Bridge"...dunno, just syain' is all.

I looked that up. There's definitely a resemblance there. Whistler is the guy famous for painting his mum, isn't he... ?

Ya, https://en.wikipedia.org/wiki/James_McNeill_Whistler, and that is him. You're right. Particularly the concept art for Dishonored is very similar.
BTW, Oof, I tip my hat to you. It's not everyday you meet someone who has read the Ghormanghast Trilogy.

We who have read him tend to be awesome people.

/awesomefive!