If you're looking for a photo of Mary Todd Lincoln, type her name, in quotes, in the search box and click on "SEARCH." You'll turn up photos of Mrs. Lincoln, others of her birthplace, statues, costumed speakers who portray her, historical markers, her grave, etc. There's one of a dress belonged to her, and it leads you to the Chicago History Museum's photostream where you'll be rewarded with other Lincoln treasures.
Any number of museums and libraries have photos and images from their collections on Flickr--from the Library of Congress to a museum that exists only in its creator's mind and his/her photostream in cyberspace!
I wish I could tell you that there is an easy way to find a list of all the museums/libraries that have collections on Flickr, but I've yet to come across it! If anyone knows, please let us know in the comments below. Mostly, I've found things by using the "search" feature, or, completely by accident. For example, I was looking at the page titled "The Commons" and there I found institutions that are participating in a program that involves the public in identifying old photos. By clicking on one of the institution logos, the one marked EASTMAN, I was brought to that institution's page where I found 804 images.
I know the Boston Public Library has a photostream, but they're not participating in the identification program, so their logo is not among the ones on "The Commons" page. If I do a search under "Boston Public Library," I end up with a gazillion photos of the library itself taken by random people, but, to find the Boston P.L.'s photostream page, I can click on the "People" tab above the search box. It's really a hit-or-miss proposition--some institutions are easier to find than others.