Really, the large print says it all: De Niro, Douglas, Freeman, and Kline. The CVs of these collected stars are pretty much awe-inspiring. This advertisement, then, is pretty much just promoting the facts of (a) these four present day old school thespians are in a movie together, and (b) they're in Vegas, for some significant event. Whether or not the event depicted therein are worthy of attention is presumably based on the fact that it involves these men.
It's an odd "in repose" type of image that works because it's a bit different and the actors are so generally likeable. The well-dressed gents all look happy and like they're having fun. Douglas seems to either be relating a story or agreeing comically with what someone just said. Here's the plot synopsis from IMDb:
Three sixty-something friends take a break from their day-to-day lives to throw a bachelor party in Las Vegas for their last remaining single pal.Which works for the poster, actually. The only problem, then, is the tagline "It's Going to be Legendary," which sounds like we should expect something like The Wedding Crashers or possibly The Hangover. The only alternative is equally-corny: that the "legendary" part is just playing off of the profile of each of the leading thespians.