Alan McDonald was arrested following a raid on his flat in Motherwell, Lanarkshire, in August last year.
The former children's voluntary worker was locked up in 2002 for a similar offence.
McDonald now faces another jail-term after he admitted, at the High Court in Glasgow, downloading child porn.
Police had seized computer equipment when officers searched the 38-year-old's home in Dalriada Crescent, Motherwell.
The court was told 258,000 still images were recovered along with 758 movie clips of youngsters being abused.
Some were at the worst end of the scale used to grade such material.
Judge Lord Pentland remanded McDonald in custody and sentence was deferred.