Let me start with a link that the prolific Scott Roberts put on the Historic Washington Listserv about a site called History Pin. Apparently there was an article in the Post about it. I've played with it a bit, it's neat. Sorta like Shorpy but linked with maps and there is an app associated with it.
Also with a few pictures is a book. Goodness I love the Internet, for it has brought forth Google Books and within the Books of Google there is Dr. George Kober's 1907 book "The History and Development of the Housing Movement in the District of Columbia" which has photographs of DC alley dwellings, tenant's homes, houses in the TC and near downtown. When looking at some photos, I like to look for litter. Unless it's New York City in the 70s and 80s, most people seem to remember the past as being cleaner than it actually was.