It says on the bottom left hand side of every page of the Shorpy website under "About the Photos" that many of the photos are from the Library of Congress and links to the Library of Congress website.

For an example of how this can work visit http://www.shorpy.com which posts many photos from the Library of Congress collections.

For instance this photo: http://www.shorpy.com/node/1771 labeled by the Library of Congress as "post office" was identified by blog commenters as Our Lady of Lourdes School at 468 West 143rd Street in New York.