This is the only Luna cap I have, but I think the one you're thinking of is from PHOENIX, during the fight at the Ministry. She ends up with a mouthful of blood, and it's basically the most beautiful thing ever.

(If it is DH, then it's probably from when they rescue her? But I'm not sure about the light in those scenes, and I don't remember her face particularly well. I do know that if I had the PHOENIX icon, I'd use it instead of this one!)