Scott Bourne, who is the owner of PhotoFocus blog, has a list of photographer you should know about and I agree with him. Below are links to these articles. I strongly urge you to check them out:
http://photofocus.com/2012/04/01/photographers-that-you-should-know-henri-cartier-bresson/
http://photofocus.com/2012/03/12/photographers-you-should-know-about-edward-weston/
http://photofocus.com/2012/02/13/photographers-that-you-should-know-about-dorothea-lange/
http://photofocus.com/2012/02/26/photographers-that-you-should-know-about-paul-strand/