The image is taken from a moment in the episode, in which Supergirl is injured and Superman has to pick her up and rescue her. It was quickly picked out by comic book fans as a nod to that issue -- in which Supergirl died while battling the Anti-Monitor.
The image is one of the most iconic covers in DC Comics history -- so much so that artist George Perez is often approached to cosplay it. Last year, they had a similar promotional image for the big The Flash/Supergirl crossover, when they reproduced an old Superman cover depicting the two characters racing (except with Supergirl standing in for the Man of Steel).
You can check it out in the attached image gallery.
IAN GOMEZ STARS AS SNAPPER CARR — CADMUS attacks National City with a kryptonite powered villain who ends up seriously hurting Supergirl (Melissa Benoist). Superman (guest star Tyler Hoechlin) blames Hank (David Harewood) because the kryptonite was stolen from the DEO. Meanwhile, Kara's first day at her new job doesn't go as planned after she meets her new boss, Snapper Carr (guest star Ian Gomez).
Glen Winter directed the episode written by Robert Rovner & Caitlin Parrish.
Besides Gomez, this season brings in Tyler Hoechlin as Superman (appearing in the first two episodes, and maybe more), Chris Wood as Mon-El, Sharon Leal as Miss Martian, Lynda Carter as the President of the United States, Floriana Lima as Maggie Sawyer, Katie McGrath as Lena Luthor, and more. As Kara's life undergoes some serious changes in the wake of the events of Supergirl season one, she's got to deal with the arrival of what appears to be another Kryptonian in the form of Mon-El.
This episode's kryptonite-powered villain is Frederick Schmidt's Metallo, as previously reported, who also plays the principal antagonist in the season premiere.
Supergirl airs on Mondays at 8 p.m.; The Flash on Tuesdays at the same time, Arrow on Wednesdays, and DC's Legends of Tomorrow on Thursdays. The Flash will debut its new episodes on The CW starting October 4; Arrow, October 5; Supergirl, October 10; and DC's Legends of Tomorrow October 13.