Here's a badly-scanned image of one of the four double-page splashes in today's Justice League #23, the climactic chapter of the "Trinity War" storyline:
If so, then you've probably read DC's 2012 Free Comic Book Day offering DC Comics—The New 52 FCBD Edition, which included this four-page, gatefold image drawn by Jim Lee as part of Geoff Johns-written story:
Robot 6 or ComicsAlliance, to randomly pick to comic book blogs I contribute to).
I think there's a couple of significant differences worth pointing out, aside from the fact that the Reis image from the mid-2013 JLA #23 is only two pages and includes about ten more heroes than the four-page, early 2012 image from Lee, some of which may hint at things that have changed about the story and the comic books it played out in during the year and a half or so since the two artists drew the two images.
1.) Lee didn't draw the one that appeared in JLA #23; I wonder if, in 2012, DC expected Lee to still be drawing Justice League by the time "Trinity War" reached its climax? Because he only lasted about two story arcs (the initial six-issue origin story and "Teh Villain's Journe"), with some fill-in issues between them.
2.) The sheer number of heroes involved has grown considerably; it looks like the original image reflected a fight between two rather than three Leagues (Deadman is the only member of Justice League Dark in the Lee image), and the Justice League of America line-up might have consisted of different characters at one point (Of the JLoA, Katana, Catwoman, Steve Trevor, Stargirl and Martian Manhunter are all missing).
3.) Mera is in the Lee image, but doesn't appear at all during "Trinity War"...save for about a two-panel cameo, including her appearance in the Reis image. That's not Mera in the Reis image, though; it's Martian Manhunter who apparently decided to take Mera's form to sneak up on and clobber Aquaman rather than just, you know, turning invisible. I assume the scene was written to explain Mera's presence in the Lee image.
When Justice League first launched, it seemed like Mera was one of the characters who were supposed to join the team shortly after the first arc: She appears on the cover of the first issue of the series, along with Deadman, a male version of The Atom that has yet to be introduced into The New 52, Element Woman, Hawkman and a female character identified in interviews as Will Eisner's Lady Luck, whom I don't think has ever appeared yet either.
Of course that cover image, which was used on two of the eleven variant covers for Justice League #1, also featured an earlier version of Wonder Woman's redesigned costume that didn't actually make it into any of the comics either.
4.) The Jim Lee image features an obviously male version of The Atom, while The Atom who would join the Justice League and appear in the Reis version was female and had a fairly different costume, which allowed her long hair to flow out the back of the cowl.
5.) Captain Marvel/Shazam's costume in the Lee image—that is Cap and not Black Adam, despite the colors—is wearing a not final version of his redesigned costume. The one in the Lee image has the more traditional collar instead of the hood that he would ultimately end up with.
6.) Superman's looking awfully healthy in that Lee version, rather than as gray as he does in the Reis version. In fact, looked-at completely out of context, the Reis version looks like it could be depicting Bizarro in a backwards Bizarro costume.
7.) Released in May of 2012, the Lee image was the first appearance of both Green Lantern Simon Baz (who debuted in September 2012's Green Lantern #0) and the New 52 version of Vibe (Justice League of America's Vibe #1 launched in February of 2013).
Clearly Johns has been working on this storyline for quite a while, and had a pretty good idea of what was going to happen at the end, at least in so far as a few key elements of the action were written in one form or another quite a long time ago (Batman having the box, everyone fighting regardless of team affiliation, Deadman attempting to possess Shazam, Superman smashing the new Lantern with a column).
And just as clearly a lot has changed since Lee put pencil to paper to draw that gatefold image teasing "Trinity War."