I happened across the blog Biblio Odyssey a while ago. It is a collection of historical images from all over the world. Some very, very fantastic stuff. I was wondering if anyone else perused through sites like this one.
My current campaign is greatly inspired by a variety of HBO TV shows - esp. the Wire, Rome and Deadwood (w/ a little 1st season of Sopranos thrown in).
As far as images go, Hieronymus Bosch's paintings have inspired numerous scenes, characters, and creatures in my D&D games. This is especially true of his depiction of Hell from the Garden of Earthly Delights triptych, but also of parts of his two The Last Judgement triptychs, among others, all viewable on Wikipedia.
I think the key to finding inspiration is just deciding that you need to find something inspiring.
Usually for my games I have very clear ideas; however I don't like that. I want something random that is off just a bit that could take the story in a new direction. What I do to achieve that goal is pick two or more sources and try to combine them. If that doesn't mesh well, then I toss some and start again.
I don't think the sources are important though I prefer simple non-complex to start. I find trying to weave complex sources to be a waste of time since the players are probably going to do their own thing.