I seriously can't fault the people who are responding to this question with the groundbreaking artwork of Tony Diterlizzi in the Planescape books. The moment I opened one of the books in my local gaming shop (back in '94, I think the shop might have been "The Tin Soldier" or maybe "Napoleon's"), I knew I'd have to run a campaign there.
But lots of people have gone that way, so let's dig a little deeper.
The early Vampire the Masquerade stuff was really evocative, in the sense that you were seeing mean bad-ass brooding types, and this was an "awesome game where you could be a mean bad-ass too". Most of the other early World of Darkness stuff had that kind of vibe happening too... the illustrations in Werewolf were more primal and violent, the imagery from Mage was more heroic and surreal. I'm not going to get into the debate about whether the games themselves actually supported this style of play, because today's question is about the art... and whether you found that art inspiring.
Then there are the beautiful books that I really wanted to be inspired by, but they just felt like they were trying too hard. A specific case in point was Nobilis, I wanted to love it, and I know there are fanatical devotees of the game, but it just didn't do anything for me. Perhaps I'd already played Amber and Changeling: the Dreaming, and this just felt like a pretentious hybrid of those.
I was going to instantly dismiss any game that used predominantly photographical imagery, because most of them are licensed properties simply using screenshots from the TV-show/movie from which they derive. If I'm inspired by those images, they probably inspire because of their context, and they probably inspired me more when I saw them screened as a part of the show. I will make a notable exception for +Josh T Jordan's Heroine though. Even though I found the rules of the game to be a bit to abstract and simplistic for my tastes, flicking through the book made me want to tweak the rules a bit and play a session or two.