Sohail Ayaz, a leading member of an international child abuse network, came to the UK on a two-year visa after molesting and photographing a 14-year-old boy.
Ayaz, who is also wanted in Italy in connection with a Romanian child sex ring, was arrested at the charity's headquarters earlier this year.
Officers had been concerned he might find a way to use his job to target 'some of the world's most vulnerable children' but their fears proved groundless.
The 35-year-old's home in Barking, east London, was searched and thousands of child porn images recovered. Some depicted six-month-old babies.
A number of the images fell into the most serious Level 5 category, and showed 'distressed' youngsters struggling against 'sensory deprivation' as they were 'tied up, blindfolded, suspended' and subjected to sado-masochistic acts.
Further inquiries discovered the Pakistani national had also abused a young boy in another country who cannot be identified for legal reasons.
He forwarded pictures of his assaults on the youngster to a fellow paedophile in Britain, London's Southwark Crown Court heard.
Ayaz pleaded guilty to three sex abuse charges related to the the youngster, two of indecently photographing him, and one of distributing the images.
He also admitted two counts of possession - one involving 397 'viewable' indecent pictures of children and the other 112 video clips, all of which had had collected through 'file sharing activities'.