I'm a long time making pictures (6)
I believe the answer is:
images
'pictures' is the definition.
('image' can be a synonym of 'picture')
'i'm a long time' is the wordplay.
'a long time' becomes 'ages' (ages can mean a very long time).
'im'+'ages'='IMAGES'
'making' is the link.
(Other definitions for images that I've seen before include "Reflections in mirrors" , "Visual counterparts in lens or mirror" , "Likenesses or impressions" , "Mental representations or figures" , "Pictures; metaphors" .)