Shouldn't it be "used for" to express a purpose of using something? what's different between "used in" and "used for"?
Both "used in" and "used for" are correct in this context. "Used in" is correct here because "in" is used to describe something that is enclosed by or inside of something else. "Print media" refers to printed publications such as newspapers, magazines and books. Since "print media" are enclosed objects (because print media has pages), a photo can be "in" or "inside of" print media. "Used for" is correct here because "for" implies the purpose of something, and here the purpose of the photo is for it to be in "print media".