Battle Avatar Appearance

So a minor question when compared to all this talk of raids, but I'm curious: is there a rhyme or reason for what a player's battle avatar looks like (i.e., the picture that shows up on the vs. loading screen when you battle an enemy gym)? For instance, my avatar is pictured front on and is holding a pokeball, but the wife's is just her character in silhouette and no pokeball. Is this based on a person's level, stats, or anything or is it just random?