If you are telling a story about when you took the picture, then describe the events in the photo in the past tense ("in the picture, my boy was smiling... "). If you are looking at the photo now, you use present tense ("As you can see in this picture, the boy is smiling..."). By the way, "delight" and "glee" are synonyms that mean the same thing, so you can't use one as an adjective for the other. The boy is either "smiling gleefully" or he is "smiling delightfully."

We tend to use Present Continuous to describe photographs. We are wearing ..., I am holding ..., He is smiling .... The logic is that the photograph captures a kind of permanent 'now', a frozen moment.