That's one I keep seeing, though that may be because I'm paying attention to Civil War -type sites and posts.
One reason I like it is that it almost looks like Williams and McClernand have just shared a joke-- McClernand's expression in this photo has always looked peculiar to me, and now it actually seems to make sense!
(My wife notes that Pinkerton, in particular, looks about as un-natural in the "hand in the coat" pose as anyone ever photographed.)