Police have appealed to parents to examine the Halloween pictures they took of their children at Southwark Park to see if a killer is in the background.
The 38-year was fatally stabbed by a masked attacker on 31 October.
His attacker is thought to have been waiting for Mr Djelal for 15 minutes, before stabbing him twice.
The attack on the father-of-one lasted less than 30 seconds.
Mr Djelal had convictions for violence and had only been released from prison in July, leading detectives to consider whether the murder was a revenge attack.
No one has come forward to say they planned to meet him in the park, meaning the killer might have lured him there.
CCTV footage captured images of the man walking calmly near the scene close to a children's playground and a school.
The man, believed to be aged between 20 and 35, was wearing dark clothing with a hood covering his face but had a fluorescent hi-vis vest over the top of his clothing.
Detective Chief Inspector Simon Harding, from the Homicide and Major Crime Command, said he hoped someone who was in the park at the time may have seen or inadvertently pictured the attacker.
"The park itself was quite busy that day, it was the middle of the day and there were quite a few people around with either kids or jogging or walking dogs, and because it was quite a nice day people would have taken a lot of photographs or video of their kids," he said.
"It was also Halloween, so people would remember being there."
Anyone with information are urged to call the incident room on 020 8721 4205 or Crimestoppers anonymously via 0800 555 111.