I think your husband is lying about them just being friends. The joking and convos with his friends and all other details point to him having more than platonic feelings. There is no reason for him to have a butt picture from her. That’s sexually suggestive. Seems he did emotionally cheat on you. So what do you do now? Can you trust him again? He should have deleted her photos out of respect for you but refused. Clearly he cares a lot more for this woman than he lets on. I wouldn’t tolerate that for one minute. If it were me I’d be separated at this point. He is lying to you.