Pictures in large book needing effort (8)
I believe the answer is:
toilsome
'needing effort' is the definition.
(requiring toil)
'pictures in large book' is the wordplay.
'pictures' becomes 'oils' (oil paintings).
'in' means one lot of letters goes inside another.
'large book' becomes 'tome' (a tome is a large or heavy book).
'oils' put into 'tome' is 'TOILSOME'.
(Other definitions for toilsome that I've seen before include "Needing much effort" , "laborious" , "needing much work" .)