The terrified fanatics are photographed with their hands cuffed behind their back with the message: “Kill him or let him go?”
Members of the public then have an hour to vote.
Most of those taking part vote to execute the IS prisoners and grisly photos are later added showing them dead.
A guerrilla group called Iraqiswat is behind the chilling new tactic as they seek to match the savagery shown by IS in Iraq and Syria.
Photos seen by the Sunday Express show a bloodied, long-haired man, said to be an IS fighter, awaiting his fate.
A message from Iraqiswat, which has a skull as its emblem, later confirms: “As you wish my followers. We will kill him.”
Another image is then uploaded showing him dead. Experts from the Henry Jackson Society believe the photos are genuine and appear to be from a Shia militia group that includes former members of the Iraqi Special Forces.
Research fellow Kyle Orton said: “I think this is probably credible. While this is an official uniform, this is almost certainly from an Iranian proxy militia.
“I have not seen them asking for a vote on people’s fate before and there is no guarantee this guy they have is an IS member. The militias treat any military-age male who remained under IS rule as a collaborator.
“But atrocities by militia-infiltrated security forces or militias in official uniforms, as is often really the case now, have been really common, as has disseminating the pictures and videos.
The messages are: ‘We are tough enough to face IS,’ which is a message to their own Shia constituency and also designed to terrorise Sunnis generally so they will stop rebelling against the Iraqi government.”
During the operation to push IS out of Tikrit in March and April, some of the militia were officially members of the Iraqi security forces, including elite units like the SWAT and Special Forces.
Mr Orton added: “They committed horrific atrocities – torture, massacres, beheadings on a scale to match IS, and they photographed themselves doing it.
“From 2009 these militias launched a campaign of murders against homosexuals and these videos were circulated on mobile phone apps.”
US officials said last night that about 180 IS fighters were killed in air strikes last week after they began attacking Iraqi Kurdish Peshmerga positions on Wednesday night in Mosul, northern Iraq, using machine guns, car bombs, rockets and armoured bulldozers.
The failed attack was the biggest IS assault in five months.