So I want to make it so the character picture changes when HP hits a certain % but I'm not finding any thing that allows me to condition branch the HP;

Picture 1 shows when HP is 81-100%
Picture 2 shows when HP is 65-80%
Picture 3 shows when HP is 26-64%
Picture 4 shows when HP is 25% and below

Im not for certain, but I think you have to put that into every battle you design for the game. We had an item in a game that had a 10% chance of activating when the characters HP fell below 10% and the only way we got it to work was through putting into every battle.

If you're going to do it with scripts, just put it under Game_Actor#refresh, which is called basically all the time whenever something happens.