Sentencing Rigby, Mr Justice David MacKay told him:
The jury has convicted you of the murder of this little boy. You don't need me to tell you what a terrible crime that was.
You abused that trust by punishing him physically in quite a revolting way.
It's a chilling, terrible picture. He suffered considerably before he died, I am satisfied. He was a vulnerable boy. You abused your trust.
The judge said he did not think Rigby intended to kill Rio but had a "sudden loss of temper" and "set about him, inflicting the fatal blow".
More top news
Largely dry, wind and rain arriving to the northwest tonight
Heartbreaking notes show Ayesha Ali's despair as she tried to please her mother, who - with her lover - tortured her for being "evil".
The Ed's Diner customer was left a little green after discovering he was sharing his meal with something that was not on the menu.