Public websites of museums, corporations, and commercial ventures that have archives of photos available for online viewing.
Searching by name of photographer is one of the most efficient ways to search the NUCat library catalog. By searching the library catalog you can find books by or about that artist, most of which will be located in the Art Library.
Here is a list of some influential photographers and the time periods in which they did most of their work. It is by no means a comprehensive list, but it should help to get you started.
Early 20th century
ARTstor has thousands of images to search. You might want to start by browsing through these collectioons:
Oxford Art Online has biographies of photographers and links to samples of their work. This will help you identify artists and art movements with works that resonate with you.
The article on Photography, especially section II--History and Influence, discusses different time periods and has links to biographies of photographers.
Books in the Northwestern libraries with substantial photographic content will contain the subject category "pictorial works". For example, find photographs of Ireland with the Subject Heading search: Ireland pictorial works. Or try a keyword search, e.g. trains and pictorial, for books with photographs of trains.