Why not just tell your girlfriend what you told us? That you actually took their stuttering as a compliment because you thought it was due to their positive reaction of the photo. And that you felt good in that moment, not bad. However remind them again that they can stutter around you however much, and that you love your guys’ conversations, and the stuttering is something you see as a neutral thing. It’s neither a bad thing or a good thing. But it’s a part of them you find unique and you appreciate them being themself around you.