There are a few different sets of rooftops on the Dundjinni forums. They are a really good place in general for detailed pngs.
The problem with detailed images though is that unless you have a lot of different ones the repetition begins to grate with a large city. I think the best plan is to make varied believable outlines in vector format so they rotate and scale without degrading. The outlines are more abstract and the mind is ready to accept their repetition more. IMHO.