For the most part, 19th century photography is portrayed accurately in this episode. The photographer would certainly have removed the lens cover and counted before covering the lens again, and the subject did have to hold still for several seconds - 3 to 30 depending on light conditions and how "modern" the equipment was. By Lucas' time, although photographic equipment had made considerable advances in the 20+ years since the Civil War, there still would've been many photographers who were using older equipment.
However, because of dust and motion blur, it is unlikely that a "portrait" would be taken on the street. Also, a "wet plate" (which appears to have been used here) had to be prepared just before the shot and then developed immediately, before it dried. Developing such a picture at a later date was an unlikely option. (thanks to Margie at rifleman-connors)