Picture book theory that's used in magazines (15)

I believe the answer is:

photojournalism

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'that's used in magazines' is the definition.
I can't judge whether this defines the answer.

'picture book theory' is the wordplay.
'picture' becomes 'photo' (a photograph).
'book' becomes 'journal' (journal is a kind of book).
'theory' becomes 'ism' (I have seen 'Distinctive theory' mean 'ism' so perhaps 'theory' could also mean 'ism').
'photo'+'journal'+'ism'='PHOTOJOURNALISM'

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(Another definition for photojournalism that I've seen is " snapping at the heels of the rich and famous?".)

I've seen this clue in The Telegraph.

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