Pictures one needs to see one million times (6)
I believe the answer is:
images
'pictures' is the definition.
('image' can be a synonym of 'picture')
'one needs to see one million times' is the wordplay.
'one' becomes 'i' (Roman numeral).
'needs' means one lot of letters go next to another.
'to see one million' becomes 'm' (abbreviation).
'times' becomes 'ages' (age is a kind of time).
'i'+'m'+'ages'='IMAGES'
(Other definitions for images that I've seen before include "Pictures, representations" , "ideas" , "Pictures or photos" , "Photos or representations" , "Reflections in a mirror" .)