I stumbled upon a few of Dave Simons original AD&D 2nd edition Monster Manual illustrations a while back. My favorite amongst them is the iconic Hell Hound, referenced on page 187 of the
MM.
Please excuse the cell phone pic, but you get the idea.
On the top it says #148 HELL HOUND in black
On the bottom in pencil it says, Rust or Red Brown Fur Red Eyes Black Teeth
Then 70% in blue pen in the corner
Holding the original to the
MM page, yes, the published copy looks to be 70% the size of the original. The paper is off white card stock (looks dark in the picture due to poor lightning). It's not signed but neither are his illustrations in the
MM.
Several of the demihumans were done by DiTerlizzi and I've seen his originals from the
MM go for $15k. Not sure what Simons is going for these days.