Anyone recognize these officers?

[Championhilz]

I am trying to identify these two Union officer images in the collections of the Mississippi Department of Archives and History - the only clue I have is that they both served in the Vicksburg Campaign. Any help will be most appreciated!

 

[W Charles Young]

Maybe (I stress maybe) John D. Stevenson for the top one.

 

[observator]

Colonel or major?