Come on, it's fantastic how they picture your feelings (9)
I believe the answer is:
emoticons
'they picture your feelings' is the definition.
(images which represent emotions)
'come on it's fantastic' is the wordplay.
'fantastic' indicates an anagram.
'come'+'on'+'its'='comeonits'
'comeonits' anagrammed gives 'EMOTICONS'.
'how' acts as a link.
(Other definitions for emoticons that I've seen before include "symbols" , "facial expressions" .)