Here is a GIF of the five nominee posters for Best Visual Effects.
Interestingly, the poster for "The Revenant" that you see above was not the first version published. In early January, when the posters were originally published, the poster for "The Revenant" featured a frame of original photography in the upper right corner: a stunt performer clothed in a blue jumpsuit, wearing a bear head. All of the other shots on all the posters were final visual effects shots. A few weeks later, a new version of "The Revenant" poster was published, featuring a final frame of a landscape shot.
Most likely, the person in the blue bear costume was stuntman Glen Ennis, who "played" the bear during live-action shooting. The bear, as you may know, was ultimately realized entirely with computer graphics by Industrial Light & Magic.
An excerpt from the Ennis interview from Global News:
How does it feel to be involved in a movie that just won Best Picture at the Golden Globes, and is nominated for a Best Picture Oscar?
It feels cool. I’ve been in a lot of movies and had parts that I thought were going to be substantial; sometimes they are, sometimes they’re not. When I was doing it, I had no idea that the bear scene was going to be such an integral part of it. It was hard, sweaty work for me. And now that it’s turned into what it’s turned into, I’m incredibly proud of the work. I’m a small part — obviously the geniuses who made it look as good as they did with the CGI deserve credit — but it’s still pretty cool to be a part of it.
At first glance, one might suspect the reason for the poster change could be part of director Alejandro Innaritu's intention to keep the movie magic behind the visual effects of the bear a secret. I wrote extensively about this in my Predictinator article (jump to "Potential Spoilers"), and have also tweeted about it. Upon further reflection, however, it appears as though the 'blue bear' image was added by mistake, since every single image in each of the posters is a final visual effects shot. No other original photography, wireframes, or any other "before" photo appears. The newer "Revenant" poster merely fixes that mistake.
UPDATE: 2/23/2016: On Twitter, Gavin Graham points out that the "Ex Machina" poster contains concept art.
@tvaziri not all of the images on the academy VFX posters are finals - Ex Machina contains concept art.— Gavin Graham (@gavgraham) February 24, 2016