Google has a special site dedicated to the hosted images and on the site it is explained how to search this archive from any photo search. This could be a great resource for historical research from the 1840's on. I know our school spends quite a bit of time on civil war, immigration, presidents which are all covered in the archive.
Here is what the site says,
Search millions of historic photos
Search millions of photographs from the LIFE photo archive, stretching from the 1750s to today. Most were never published and are now available for the first time through the joint work of LIFE and Google.