Detectives believe the man may have seen what happened to Richard Bowes who was attacked as he tried to stamp out a fire in Ealing, west London, on Monday.
The 68-year-old local man died from head injuries three days later.
A boy, 16, has been arrested on suspicion of Mr Bowes' murder. A 22-year-old man has been bailed.
The 22-year-old, who was held on suspicion of murder, was also questioned on suspicion of rioting and burglary while the boy is also being held on suspicion of burglary.
A 31-year-old woman is being held on suspicion of perverting the course of justice.
In a direct witness appeal on Wednesday, the Metropolitan Police asked a person captured in a CCTV image to contact them.
Mr Bowes, of Haven Green, Ealing, is thought to have been attacked after confronting teenagers setting fire to two industrial bins in Spring Bridge Road.
A witness said rioters attacked the pensioner when he tried to put out a fire started in a supermarket bin near the Arcadia shopping centre.
Detectives investigating his death have urged anyone with footage of the area at the time of the attack to contact them.
Meanwhile the Richard Mannington Bowes Relief Fund has been set up to help people living in the area who were affected by the riots.
A potential witness in the case of a man who was fatally injured during the London riots has come forward, police have said.