Check Google, the phrase "on that photograph" shows up in blog posts, song lyrics, and even books. Why is it not accepted here?
It's not idiomatic.
"On that photograph" is a correct phrase, but has a different meaning, in that there's a physical object on the surface. Like a smudge.
It's a common mistake, though, especially among non-native English speakers, since literally every other European language I encountered deals with a picture's contents with its form of "on".
Why is the translation "Is it you in that photo" not correct? Isn't photo a common short version of the "photograph"?
Photo, picture and image should all be accepted translations (they are not in this exercise) - photograph is too formal for an informal conversation suggested in this sentence.