This question must of come up in the past several times.
In old PHOTOS frequently the man is posed with his hand inside his coat.
WHY?
What was the significance of that pose?
It was the style of the time.
They wore turnips on their belts too.
Those old clothes were itchy. 
I think it was just a matter of style. to make a formal pose. Photography was a new (and expensive) media, so people wanted to appear impressive, and serious.
Notice, too, that people in old photos are never smiling…maybe for the same reasons.
Another factor in old photographs was that the exposure time for the process (wet plate collodion) used in the 1860s was very long. People posed formally for photographs because they had to hold the pose for several seconds while the plate was exposed. The technology available at that time did not make “snapshots” possible.
I think yabob has the answer, as that was going to be my guess. There were also lots of iron frame contraptions to hold the head and body still. (Some exposures could take several minutes in those day, even in bright light. Films then had what we would refer to as ISO ratings in fractions.)
In the case of Napoleon, I had always heard (thou it could be BS) that painters of the era didn’t do hands very well. (BTW, your second link doesn’t work.)
yeah, but why the hand inside the coat instead of hands folded down in front of them. The inside the coat pose kind of looks like they’re reaching for a pistol.
He’s soothing the pet gerbil tucked into his vest pocket.
*Et voilà*. It’s a revival of an eighteenth-century tradition/fashion. They were trying to look like they were well-bred gentlemen. No gerbils, no hand deformities, no tired arms from long exposure times.
The hidden Bowie knives, to be used if that Fiend, the Photographer, tried to use his Devilish Device to steal their souls… 
Onions, too, only you couldn’t get white ones because of the war. The only thing you could get was those big yellow ones.
This reminded me right away of the famous Napoleonic pose ante cameras.
Perhaps this site is helpful
:smack: I knew turnips wasn’t right. Oh, well.
Because the photographer told him to. Seriously, there are poses that photographers tell their clients to adopt, and the subject follows along, figuring the photographer knows best. Portrait photographers still do it today; the clichéd chin resting on thumb and forefinger, woman wearing cowboy hat, holding brim between thumb and forefinger. There are whole books on the subject for photographers; here is a website with standard posing techniques. They run in periods of fashion, and the in hand in coat was “in” in the 19th century.
Unless the photo subject is a Scotsman, in which case he’s got hold of his sgian achlais. 
And here I always thought they were reaching for their guns.
all were members of the mob?
D’oh! Strike that.
Damn thee Sunspace
And, it cost a nickel to get your picture taken, and in those days, nickels had bumblebees on 'em.
If that was common of everyone in every picture back then, why did it become associated with Napolean of all people?