The incident happened in December when the 18-year-old man suffered what was described at the time as a broken jaw.
Police want to trace a woman and a man.
The woman is described as being in her late teens to early 20s, with long brown hair. The man is thought to be in his early 20s.
The woman was wearing a beige-coloured jacket and beige knee-high boots.
The man was wearing a dark blue puffy jacket, blue jeans and grey and black trainers.
PC Graham Crawford said: "This is a serious incident resulting in a male sustaining a severe facial injury.
"Violent behaviour will not be tolerated and I would urge anyone who recognises the male and female pictured, or has any knowledge of this incident to come forward and contact the police."

Eight-year-old Garfield, who makes the trip to Sainsbury's in Ely in Cambridgeshire on a daily basis, has his own Facebook page created by fans.
Owner David Willers posted a message asking shoppers to lay off the treats as a vet found Garfield, who weighs 6.6kg (one stone), was overweight.
"Everyone loves him," Mr Willers said.
"The other day when it was cold and he stayed indoors, a member of Sainsbury's staff messaged me to ask where he was, which was very sweet.
"He brings more customers into the shop - people tell me they go and shop there just to see him."
Garfield's Facebook page was set up last summer by a couple who had spotted him in the shop, and is now run by Mr Willers.
"We worry about him crossing the road, but we don't really get any negative comments about his habits," he said.
"We live about a four-minute walk away from Sainsbury's but Garfield knows all the shortcuts so can get there quicker than that."

Brittany Carter, 23, is accused of performing a sex act on the 29-year-old driver while Corey Jackson, 20, allegedly held a knife to his throat.
Police said the pair then stole $32 (Â£26) from the victim before fleeing the scene in the city of Findlay.
"We don't know why she did it," police Lt Robert Ring said.
"Maybe it was a distraction because they took money from him."
The assault unfolded after Ms Carter called the Trinity Express Cab Service in the early hours of 28 January, according to police.
She was arrested after the driver reported the incident.
Ms Carter faces first-degree felony rape and aggravated robbery charges in Hancock County.
She was indicted twice on drug charges in 2016 and is accused of heroin possession.
Mr Jackson remains at large and a warrant has been issued for his arrest, say police.
He is charged with aggravated robbery and complicity to commit rape, both first-degree felonies.
Police investigating an "unprovoked serious assault" on a teenager outside Stonehaven railway station have released CCTV images of a woman and man they want to trace.
[NEXT_CONCEPT]
The owner of a cat which visits a supermarket every day, has had to ask people to stop feeding his pet because he was putting on weight.
[NEXT_CONCEPT]
An Ohio woman has been charged with raping a cab driver at knifepoint before she and another man robbed him, according to police.