Prisoner putting up pictures on the other side (6)
I believe the answer is:
contra
'on the other side' is the definition.
(I've seen this before)
'prisoner putting up pictures' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'putting up' says the letters should be written in reverse.
'pictures' becomes 'art'.
'art' back-to-front is 'tra'.
'con'+'tra'='CONTRA'
(Other definitions for contra that I've seen before include "Con, Art could be against it" , "Rebel fighter" , "right winger" , "US-backed fighter" , "on the other side" .)