The photo might have been of her torso, with her hands across her modesty, here being a humourous euphemism for her breasts.
When you looked up 'modesty', did you expect to find a long list of all the body bits (including bottoms!) and whatever else might need to be veiled to preserve public sensibilities?
Here, the writer used 'modesty' and we understood what he really meant. It is only up to the vocabulary and skill of the writer and perhaps some other word could have been used and we would have smiled, knowing what was meant. I am no such writer, but another word that could equally have been used would have been her 'shyness'.
Student or Learner