Day one of the Bulge?
Lt Lyle Bouck Jr. Found his platoon, the Intelligence and Recce Platoon of 394th Infantry Rgt (18 men, accompanied by 4 artillery observers) in Lanzerath, right in the path of the Germans. Held off a battalion for nigh on 24 hours, buggered up the German timetable, inflicted somewhere in the region of 100 German casualties before being finally outflanked and taken prisoner.
Because of their being taken PoW, the story of what the platoon did took years to properly emerge, and Bouck - and everyone else in the platoon - was decorated in 1981; Bouck getting a Distinguished Service Cross.
Not tricky to find a photo for someone to be representative of Day 1...
The trick, though, is to find a photo for someone on the very first post of Day 1, which is relevant to the story being told. Firstly, the post started out before Peiper gave the 'go' command, with his diary entry, so Lt Bouck has had no reason to hold off anything at that point. (The original post can still be viewed under the edit history, they were obviously going for a 'story-telling' vibe). Further, the Facebook page in question is that of the XVIII Airborne Corps, and what was important to the airborne units on 16th Dec was that the Germans had just slammed a couple hundred panzers into the US lines. What 394IR was doing kindof wasn't all that important to the paratroopers. The fact that Peiper was leading an operation which would unsettle the US as a whole, and lead to the redeployment of the Airborne, was a bit more relevant to the Airborne.