· the K. Ross Toole Archives at the University of Montana’s Mansfield Library ;
· the Denver Public Library, which has one of the best collections of western history images anywhere;
· Helena As She Was, which offers an amazing collection of historic photos of Helena;
· and the Library of Congress's American Memory project.
This last site is voluminous and includes images from the Farm Security Administration/Office of War Information Collection of images from the 1930s and 1940s. (Think “Migrant Mother.”) In addition to the well-known black and white images are rare color images, from the days when color photography was in its infancy.
A nice gallery of some of the early color images from the FSA (including two from Montana) is here: Captured: America in Color from 1939-1943 Photo Blog.