The Milwaukee Public Library has been working on its own digital collections over the past few years. If you haven't checked those collections out, make sure that you do. They are full of great historical photos of Milwaukee.
Other collections of great historic photos exist at other institutions. The Wisconsin Historical Society, in particular, has great digital photo collections relating to Wisconsin History.
With over 35,000 images from all over the state, this is a wonderful collection.
Search their collection to see what other historic photos you can find.