Pervert: David Mellor was jailed for six years
David Mellor, 62, didn't spot he had included it when he photographed the underage girl, in his kitchen and posted the picture on the internet.
But officers in Lincoln recognised the sign and identified the street and his home from the picture's angle.
The image was seen on a file-sharing website by a specialist officer with the Child Exploitation and Online Protection unit.
When police raided Mellor's property they found more then 8,000 indecent images of children on a computer.
He was jailed for six years at Lincoln Crown Court after admitting 18 sex offences involving indecent images and also breaking a court prevention order.
He had been jailed for three years in 2006 over child abuse crimes.
Prosecutor Andrew Scott said: 'The officers must be commended for excellent detective work using their local knowledge.
'When police entered the property it was obvious where the photograph had been taken.'
The court heard how Mellor broke his court order by becoming friendly with two young girls under the age of 13 in West Yorkshire after his release from prison.
When he got the youngsters alone, he asked them to remove their clothes before taking snaps.
One visited him for several days after he moved to Lincoln. Judge Sean Morris branded Mellor 'dangerous'.
Peter Davies, Chief Executive of the online unit, said: 'Offenders may think their activities are anonymous online but this is simply not the case. They can and will be found and have to face the consequences of their actions'
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