The photograph shows Hannah Witheridge, 23, and David Miller, 24, celebrating with friends including Christopher Ware, at a bar on the island of Koh Tao where they were mysteriously murdered and left semi-naked on a beach.
Looking happy and carefree, the couple appear to be with a large group of friends, enjoying their evening with drinks at a bar called Choppers.
It is understood the photo was taken three hours before they were killed.
Thai police investigating the horrific crime say they are close to making an arrest and said they would arrest and charge two men by the end of this week.
Detectives have widened their investigation and are moving off the island and onto the mainland to track down the suspects, the Mirror reports.
They are hunting for a 22-year-old man who left Koh Tao soon after the killings, who is the son of a prominent businessman.
A police source said: “We want to trace this man as a matter of urgency. We believe he was on the island when the crimes were committed and left shortly afterwards. We think he is in the Bangkok area.”
Their search for this man comes after the Mirror revealed the murdered couple got into a fight at a bar with a local gangster.
Thai police said they carried out a DNA test late Sunday on a 23-year-old Thai speedboat driver arrested for a drug offence.
“We are waiting for the (result) of his DNA test... It is not clear if he is a suspect yet” in the murder case, Kiattipong told AFP, adding that the man drives a speedboat between Koh Tao and the nearby island of Koh Samui.
Police are also searching for two friends of the driver, Kiattipong said, without giving further details.
Deputy National Police chief Somyot Poompanmoung told AFP the man was being questioned but had not been arrested or charged over the murders.
Earlier Somyot said the DNA of two Asian men was found at the crime scene and had been sent to Singapore for advanced analysis.
Forensic experts in Bangkok discovered traces of semen at the scene but could not find a DNA match with the 12 people they have questioned so far over the case.
Those questioned included two of Miller’s British friends and several migrant workers from Myanmar.
The murders rocked the normally laid-back holiday island, which is popular with divers, and delivered a fresh blow to Thailand’s image as a tourist haven after months of political protests that ended in May’s army coup.
Thailand’s most senior police officers have descended on Koh Tao amid accusations — led by the British media — of a bungled investigation.
Critics have said police chased the wrong leads, failed to lock down the island and poorly secured the crime scene in the hours after the badly beaten bodies were found early last Monday.
Witheridge’s family, who travelled to Bangkok to retrieve their daughter’s body, issued a statement requesting privacy.
“We have now returned to the UK with our beautiful Hannah,” said the statement, which was released on their behalf by the British Foreign Office.
Thailand’s military ruler Prayut Chan-O-Cha — who is also the prime minister — made a rare public apology after he suggested tourists in bikinis could be more vulnerable to attack.
His comments were widely pilloried as sexist and insensitive following the brutal double murder and he has since sent letters of condolence to the British prime minister as well as the victims’ families.