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