Quote Originally Posted by Krotoan View Post
It actually seems to remember the expression too (or however the emote works) since when swapping portraits you can see the expression fall off the face as it morphs back to "none" when chosen.
From everything I've seen so far, the portraits are not screenshots but a "live" shot of your character frozen in place. Most tellingly, weapons with special effects are animated and moving even though the character is still. And of course it's able to change outfit according to what you are wearing. You can also see hairstyles pop into position as you switch to a pose and (I assume) gravity is applied to the moveable bits.

It seems that it's not storing the actual picture but the instructions to generate the picture.