We can go on about how much we think it's fake based on our skepticism alone, but what really matters is looking at already released screenshots.
At first, I thought I saw Zelda screenshots that were similar, but I was mistaken. I checked every other 3DS Battlefield screenshot I could find though. Out of all of them, only one looks to have a strikingly similar background to the solo shot of Palutena:
As you can see, the two shots are taken at near identical angles. However, even then, the bottom left part of the Peach shot is not visible in the Palutena solo shot, and the lighting is also a little bit different between the two shots.
As for the Palutena shot with Mario, I can't find any shots of Mario matching that shot. And yes, I did check the 3DS Facebook page, where they have extra screenshots. Even there, nothing matches up perfectly.
So there is a possibility this is legitimate, from what we can deduce. Also keep in mind that, compared to the likes of Mewthree, not a lot of people would go through the effort of making a fake screenshot of Palutena. Especially not one that looks so good. I can't find any tell tale signs that this is fake, unlike that one screenshot of Mewthree in Smash, that was easy to figure out and prove as fake.
The only thing that gives me any true, logical reason to doubt this is the fact that the bottom screen is not shown off. I mean, if you were going to leak screenshots like this, assuming you actually had your hands on the game for a bit, you might as well show us what's on the bottom screen while you're at it. Even if it is indeed nothing. At least to prove that you haven't just got the 3DS photo viewer with your fake screenshot up! :V