I have a photo on the profile of one of my own ancestors that is a copy of an original taken in 1847 (below), and I am quite confident it is authentic. It came from my grandmother's collection. Generally, if I see photos that are said to have been taken after 1850, I believe they are possibly legitimate, but that by no means proves they are properly identified. In special circumstances, earlier photos might be authentic.
When I find a photo of an ancestor on-line that I didn't already know about, I usually contact the person who posted it and ask where they got it. One of my most precious photos, of my third great grandfather and his wife, came from a distant relative I'd never met before a third person brought the photo to my attention, and based on our correspondence, I am very sure it is authentic.