Daniel Hall, 35, was last seen at one of the famously hedonistic parties on the island of Ko Pha Ngan along with 30,000 other revellers in February 2008.
Since then friends have been searching for the Birmingham University history graduate who had been working as a labourer in Norwich, his home town, before travelling on an extended holiday.
Detective chief inspector Andy Guy, from Norfolk Police, said Mr Hall had last been seen at an “after party” at the island’s Back Yard Bar at about midday on February 25.
The chief inspector added: “Many of the party goers at the Full Moon party would have been British and five years later would now be living and working back in the UK.”
A key line of inquiry is a picture featuring Mr Hall standing on the edge, with two men in the centre, apparently looking at him.
One of the men is topless with a goatee and shaved head.
Mr Guy said the men are thought to be British and detectives would like to speak to them as witnesses.
Officers would also be interested in speaking to any of the other people in the photograph.
After the final sighting, Mr Hall spoke to two friends on the phone but was never heard from again.
He had never been reported missing before and had no reason to vanish, Mr Guy said.
Mr Hall’s belongings were found at the Lam Son bungalow where he had been staying.
It was later established that no money had been withdrawn from his bank account since three days before he went missing.