A man took indecent photos of his three-year-old granddaughter and passed them on to a paedophile friend.
The 63-year-old was entrusted by the child's mother to look after the youngster.
When the photographs came to light, the mother was "devastated" by his betrayal, Leicester Crown Court was told.
He was jailed for nine months.
He initially owned up to the police, but then denied any involvement.
One picture showed a man's hand resting on the child's leg.
Anthropologist Professor Sue Black studied the image of the hand and concluded it was the defendant's.
It was only when faced with expert evidence the defendant admitted guilt.
The married man, from Leicester, pleaded guilty to taking 13 indecent pictures and a video of a child under 13, in April, 2011.
He admitted distributing some of the pictures, as well as other images of naked children from the internet, between April and June 2011.
All of the images were categorised as level one, the least serious category.
The defendant is not being named to protect the identity of his grandchild.
Victoria Rose, prosecuting, said the defendant's associate, who received the images, was arrested on suspicion of sexual assault.
The charge was dropped, but during the inquiry his computer was examined and indecent images – including some of the defendant's granddaughter –were found.
This resulted in the associate being prosecuted for possessing the images. He received a community order.
The defendant was also arrested during the inquiry.
It was not suggested in court he had sexually touched his granddaughter and she is not thought to have been aware of any wrongdoing when the photos were taken.
Mr Murphy said the girl's mother now distrusted people.
She described herself as "paranoid", even taking her daughter off a bus recently when a male passenger smiled at the child.
Justine Robinson, mitigating, said: "He has no previous convictions and lived a blameless life hitherto.
"He's deeply ashamed and regrets the distress caused.
"He has had no contact with any of his grandchildren since these allegations have been made."
Judge Robert Brown said: "This little girl's mother left her child in your care and you abused that trust by taking indecent photographs of her, including a short video, for the specific purpose of sexual gratification and intending to supply them to another person."
The defendant will have to enlist on a sex offender register for 10 years.
