A bit of extra info, and a very minor correction.
The tax stamps, which originally were specific to particular uses starting around October 1, 1862. Very quickly this was realized as being too difficult, and any of the stamps could be used for any tax.
The cancelling was supposed to be done by writing the users name and the date on the stamp, usually it's just initials. Because of the volume of photographs many photographers used hand stamped cancels. So even is the photographer isn't identified on the mount they can be identified from the cancel. It's not usually conclusive on a particular photo since they also usually cancelled them in advance leaving the cancel only on the stamp. Unfortunate since a stamp can be added and sometimes it's hard to tell if it was there originally. On the plus side, some handstamps include the date, and that will get the date of the photo reasonably close depending on how often the photographer cancelled his stamps and how busy they were.
Here's a nice link to a collectors site showing a number of photographers cancels.
http://www.revenue-collector.com/cgi...otographer.cgi
The tiny correction is that blue stamps were available throughout the time the photo tax was charged. Here's a blue one from 1865 from the same collection.
http://www.revenue-collector.com/pages/1438_.html
Steve B