People seem to believe there's a collection of Confederate images somewhere, and their ancestor's picture is certainly included. Truth is, the one image that might exist can be found in a descendant's home. One of the better means of locating items like this would be to attend a family reunion.
From your perspective, the best route is the one Mr. Roberts just described. Do everything possible for your website to appear in relevant searches, and let visitors know what kinds of items (like wartime images) that will make your site more interesting.
If you build it, they will come....