There are two very similar images of Civil War soldiers posing with a downed pterosaur. The other has been thoroughly debunked as a publicity shot from a TV show. This, anyway, is the better of the two. That pterosaur is impressive.
There are many reasons why it doesn't quite ring true. Go here for evidence of photoshopping.
And then, just look at the soldiers. I'm told the uniforms are authentic, but consider how prosperous and well-groomed they all are. These aren't battlefield troops. And another thing: photography in the 1860s required long time-exposures- which meant people posed stiffly and stared fixedly at the camera. These guys are too casual. They're don't look like people struggling to hold a pose for 15-30 seconds. A defining mark of Victorian photographs- and a reason I love them so- is that the people in them are just so damn intense.
Finally, is it plausible that such a sensational image could have remained hidden for a century and a half? There were six people present at the incident, plus the photographer- and none of them went to the press? I don't think so.