The Punctum is a detail that pierces and punctures the heart. It is a strong emotional reaction that is unexpectedly found; that impacts our mind/memory, making us hold our breath. The Punctum can also be described as that part of the photography one carries in memory after ceasing to look at the photo. The feelings it generates for the onlooker is “love.”
One famous example is the photograph of Alexander Gardner. Surprisingly, in terms of punctum, Barthes says “the punctum is: he is going to die”.