Portrait of ~25-year-old man sitting for a portrait in a studio (subject, location and photographer unknown).

The tall-looking subject is likely Seymour Thompson Clark or John Alexander Clark, both sons of Susan Leggett Clark. ]

The individual is often mistakenly labeled as Ezra James Clark, Mary Stevenson's oldest son who died en route to Utah on a train in New York following his a mission to England. It is demonstrably not Ezra James.

If the image is John Alexander, he would have changed his hair and sideburns since his last photo before his mission. This is very possible as to fit in with the locals. Location of the original photo would likely reveal clues to the correct identity.

If the image is Seymour Thompson, it was likely take prior to his being admitted into the Utah Insane Asylum in Provo at the age of 26.

If another photo can be found with the same backdrop, we can ascertain the studio location and dates. A Salt Lake studio would suggest it is Seymour Thompson.

More detail can be found on Ezra James, John Alexander and Seymour Thompson and this photo on FamilySearch.