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
A miniature Shetland pony has surprised its owner after unexpectedly giving birth to tiny 17-inch tall foal.
A six-year-old girl was bitten on the legs by a police dog while officers were chasing a suspect.
Taiwan has taken a new approach to covering the Labour leadership contest.