The Yale University Open Access image library.
I dont know if this has been posted before but its a great resource. Yale uni has decided to open up loads of its images for open accesss. I will have to check it that means public domain or what, but it looks like a great collection. First of all its 250,000 images and secondly, a lot of them are in a format like a museum curator would want. I.e. many of the items in there are on clean backgrounds so can be converted to top down icons. Anyway, start here...
but check out something like this:
Theres loads of maps in there too which I started out on but I found the items particularly useful. Pottery, mosaics, floor tiles, reliefs, etc all good for dungeon & adventure art.
EDIT: Holy moley - theres 29000 coins to look at. Its really mind blowing amount of stuff to wade through.