It resembles the poster for This Is the End.
Who is positioned on the left under Bucky’s gun?
I haven’t seen him in a Marvel movie yet, but there are casting rumors suggesting that he will play Sentry. If the storyline follows the comics, he could become one of the most powerful characters in the MCU.
I’m really out of the loop—what exactly is Thunderbolts?
It’s a morally ambiguous group of hero/villains that usually engage in black-ops missions. There’s no certainty about where Marvel will take this, as it doesn’t follow an established comic lineup; they’ve simply chosen characters from previous movies and shows.