She does not bury the photo under a caption explaining the joke, the outfit, the mood, and the lesson. Sometimes there is no caption at all. Maybe one word. Maybe nothing.

That leaves the face to do the work. She could be amused. She could be tired. She could be reacting to something the person behind the camera said two seconds earlier.

The viewer fills in the missing part himself. That is why he stays longer than he meant to. The photo gives him enough to start guessing, then refuses to confirm anything.