The teenager, who cannot be identified for legal reasons, had accepted a ‘friend request’ from a stranger on Facebook, but just a few days later the man - from the south of England - sent her the explicit photograph.
The girl immediately deleted the picture and blocked the man - but she did not tell her parents, who only discovered what had happened when a police officer turned up at their door.
Thankfully, the man’s wrongdoing was discovered as he was on probation for a previous similar offence.
The Louth woman told the Leader: “My granddaughter is a very independent girl and she just disappears off on her computer, like all kids do.
“This incident has upset her, she’s been badly affected and may need counselling.”
She added: “I think that it’s a bigger problem than people realise.
“Thankfully this man was known to the police already, but how many others are out there and get away with it?
“Parents should know what their kids are doing on the Internet, and children should be warned that this kind of thing goes on.
“Hopefully, if a child does receive these messages, they will take the same action as my granddaughter, and block the person who sent it.
“But children should also know that it’s alright to tell a parent, or a teacher, or someone else that they trust.”