The images were found among thousands of secret Facebook messages exchanged between Peter Hanson and his brother’s wife, Marieanne Hanson, during the course of a long-running affair.
Leeds Crown Court heard the pair also discussed Peter having sex with a girl under the age of 16.
The court had previously heard how Peter, 64, of Station Road, Wisbech, Cambridgeshire, was living in France at the time of the offending but was arrested on his return to the UK.
A search of his laptop uncovered three explicit images which were classified as being of the third most serious category.
Brent Martin, mitigating, said his client accepted he had instigated his co-accused’s offending, but he did not have a general interest in underage girls or acting on the couple’s discussion.
He highlighted Peter’s “turbulent” childhood, his low risk of re-offending and the likelihood that a prison sentence would jeopardise the future of his business, which employed 11 people and two trainees.
Judge Tom Bayliss QC said he took note of all these factors, Peter’s previous good character, letters of reference from colleagues and his wife, and his early guilty plea.
But he continued: “Nothing can change the impact that your behaviour has had on the victim in this case.
“You were driving what was being done, you were encouraging your sister-in-law.
“The offence is so serious that only a custodial sentence can be justified.”
Peter, who admitted three counts of making indecent images of a child, was jailed today for six months.
He will be supervised for 12 months upon release and must sign the sex offenders’ register for seven years.
Marieanne, 52, of Kingsley Road, Adel, Leeds, was sentenced last month after admitting two offences of taking indecent images of a child, one of making indecent images of a child and two of distributing indecent images of a child.
She was given a 16-month prison sentence, suspended for two years, and ordered to complete at 25-day programme.