Someone put me onto this great resource yesterday. RIBApix (Royal Institute of British Architects), they have put up their collection of architectural images online. This includes sketches, photographs, plans. Although there is a focus on England many many other places are also represented. I remember when I started my thesis I actually had to write to them, on paper with stamps (!) to ask for details of an image.
Here are some interesting images I have found:
A set design for an 18th century opera probably by Giuseppe Galli Bibiena
The water pumping station on the Isle of Dogs, London.
The palazzetto di Madame Stern: corner of the garden wall on the Grand Canal with gondola in the foreground, Venice, taken in 1912.