When people say "I'm so happy I could cry," the first image that often comes to mind is a person with a big, radiant smile on their face, possibly with tears of joy welling up in their eyes. This image represents an overwhelming sense of happiness and emotional overflow, where someone is so elated and moved by a joyful experience that tears of happiness are almost at the brink of spilling over. It's a powerful expression of intense and positive emotions, where happiness is so profound that it can be difficult to contain.