I am looking for an antonym of selfie, meaning a photo/portrait of others.
The ancient Greek word for self is like auto, and what I am looking for is an English word for hetero (its opposite). Do you have any idea of that?
I have the impression you are looking for something kinda cute that is the counterpoint of selfie.
The most ready antonym of self might be "other." "Otherie" doesn't have much of a ring, does it? Though, keep in mind that in academic writing, Other is a concept that is an opposite to the familiar and accepted. To Other someone is to place them outside of the bounds of accepted familiarity and has ties to complicated issues of race, gender, and sexuality.
We talk of the self as being the subject or subjective. While the other is the object or objective. Try cruising around a bit on thesaurus.com looking at both synonyms and antonyms to see if you can find one that works like selfie.
I found mark, target, subject, that, pick. None of which seems perfect.
I didn't find any easy counterparts to selfie that would themselves make a cute word to refer to a picture of another person. Maybe you will have better luck.