This page is not complete but here are a few links to get you started:

Dating photographs

http://freepages.family.rootsweb.ancestry.com/~victorianphotographs/

http://blog.familytreemagazine.com/photodetectiveblog/

http://www.cyndislist.com/photos/dating/

http://australiandressregister.org/about/

http://www.maureentaylor.com/ – Direct to Maureen, the Photo Detective Website

Book – Dating Family Photos 1850 -1920 by Lenore Frost

Victoria and Albert Museum , London