The picture above is from The Ladies of Grace Adieu by Susanna Clarke.
When I was wondering if Datlow's and WIndling's Sirens and Other Daemon Lovers had Vess illos, I found that Amazon doesn't seem to list illustrators. I have no idea if it would be worth it for them, but it would be handy.
Charles Vess is one of the major fantasy illustrators, and I'm extremely fond of his little pen and ink drawings. Unfortunately, they don't especially seem to be available online, and this may be because the detail doesn't come through well. (I think Miss Mabb is rather larger than the illos for The Faerie Reel and The Green Man.)
At Faeriecon, Vess mentioned that he tried to have an aspect of what isn't seen in his pictures. The example he gave was of having just the shadow of a tree, which doesn't seem very interesting, but I wonder if there's more unseen in his pictures than something quite so simple. Those little illos that I like so much seem better than they deserve to be-- the drawing is good and the subject matter is interesting but I can't put a finger on what's so special about them.
Admittedly, that might not be the right response to magic, but I'm curious and I don't think I'll damage it by having some understanding of it.