I guess it depends what you are looking for. Personally I like to use the D&D mini's as images for MapTool.
Grab creature images from here, and get some rooms and background stuff from here. These take a bit of editing to fit your campaigns though. Another option might be to grab the dungeon tiles and scan them in. The same would apply for the maps that come with the published modules.
One final place I go for images would be Dungeon magazine. The modules they offer in PDF usually have maps there for you. I just copy the maps from the PDF (sometimes they'll offer image files of the maps as well) and use them as needed.
One last place I could recommend is the Dungeon Tile Mapper. It offers a couple tilesets that you can use to create the map, then just make an image from it and use it in MapTool.
I've actually got a growing set of images I've put together from these sources. I've thought about putting the whole mess online for convenience and sharing, but I'm sure some of that stuff is copyrighted and I don't want to get in trouble for distributing it incorrectly.