This site was created by the U.S. Army Military History Institute
and it is a searchable database of their Special Collections.
Using this database you can see if a photograph exists of your
Civil War ancestor.
Just enter the surname you're searching and the results page will
give any photos of any soldier with that surname, along with (if
known) his regiment.
Next, you can send an e-mail with the photo ID's that you're
interested in. The Army will then send you (for free) a
photocopy of the picture, along with instructions on ordering a
duplicate photo.
An 8x10 photo costs $10.00. No more then ten photos will
be produced for any one customer per year.
http://carlisle-www.army.mil/usamhi/PhotoDB.html