Photo on a steamship: old painter (7)
I believe the answer is:
picasso
'painter' is the definition.
(Picasso is an example)
'photo on a steamship old' is the wordplay.
'photo' becomes 'pic' (short for picture).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'steamship' becomes 'ss' (nautical abbreviation).
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'pic'+'a'+'ss'+'o'='PICASSO'
(Other definitions for picasso that I've seen before include "Pablo ,,,,,,,, great late Spanish artist" , "See 1 Down" , "20th century painter" , "pioneer of Cubism" , "Pablo . . . . . . ., extraordinary 20th century artist" .)