Online picture puzzle is hell! (6)
I believe the answer is:
erebus
'is hell' is the definition.
(thesaurus)
'online picture puzzle' is the wordplay.
'online' becomes 'e' (as in e-mail or e-commerce).
'picture puzzle' becomes 'rebus' (I've seen this before).
'e'+'rebus'='EREBUS'
(Other definitions for erebus that I've seen before include "Level of the underworld" , "Be sure (anag) - Greek god" , "God of darkness - cavern on the way to Hades - active volcano in Antarctica" , "Mt. -, Antarctic volcano; be sure (anag.)" , "The abode of the dead" .)