The victim, a 36-year-old man, had been out drinking and was in Briggate shortly before 2am on Saturday January 7, when he was approached by a man.
He gave the impression he was helping him and carried him down an alley into Blayds Yard off Lower Briggate.
The suspect then turned on him and punched him repeatedly and pushed him to the floor.
He then searched his pockets and removed items. The man's wallet was found nearby but his Samsung S3 phone is still missing.
The victim received a fractured cheek bone and a dislocated shoulder in the incident and was treated in hospital.
Acting Detective Sergeant Nathan Goodwin, of Leeds District Crime Team, said: "The victim was in a vulnerable state from drinking and the suspect has clearly seen him as an easy target to rob. He was seriously assaulted and received significant injuries and we urgently need to identify the man involved."
Anyone who recognises the man in the images or who witnessed the incident is asked to contact A/DS 1075 Goodwin at Elland Road via 101 quoting log number 13170009219 or call the independent charity Crimestoppers anonymously on 0800 555 111.