Finally! Linguistically speaking, there is a sentential focus somewhere: either on the phrase этот мальчик or on the pronoun я. In spoken Russian focus is easy to notice: it bears a strong, eradicting stress. In English stress is decisive only if the stressed word follows the verb. (I am DAT BOI / This boy is ME.) To put a subject in focus in writen English you have to create a whole focusing clause: It is me who you see on the picture as a boy.
Actually, I have put emphasis on the first word of a sentence in English, many times, especially if the word is "I," and I am a native English speaker. I'm not saying I perfectly understand the rules of emphasis, but I do often hear people trailing off at the ends of their sentences, so it would make sense to me that people often do put emphasis on the subject at the beginning of their sentences, and not always after a verb. Then again, a lot of Americans are just bad at English, because they didn't study hard enough.
I could see a use for it if (for example) you were to show a picture to someone online and you're the only boy there (perhaps everyone else is old, or the picture is full of girls).
Or in case you show someone a picture of your parents holding you when you were still a kid, you would say "I'm the boy."
Furthermore, you could also say "I'm this boy" while pointing at yourself in a picture full of boys.