It’s probably the famous portrait in the country – Mao Zedong, the founder of the People’s Republic of China, gazing resolutely across Beijing’s Tiananmen Square, writes Asia Correspondent John Sparks.

The oversized 6 by 4.6 metre picture has loomed over the northern edge of the square since 1949 – although the authorities have to repaint and rehang it every single year due to the effects of the weather and air pollution.

Chairman Mao (as he was better known) was a controversial figure of course. He has been blamed for the deaths of millions – but he’s still revered by many in China. So much so in fact, when people organise weddings and business events they often invite him along – well fake Mao’s anyway.

We met one professional “rent-a-Mao” called Dong Fang Zi who performs at 50-odd events a year. Mr Dong is following in a long tradition in China. It’s called “tishen” – or body replacement – and this particular actor sees it as an honorable profession. “Mao lookalikes are so popular because the image of Chairman Mao is deeply rooted in the heart of the Chinese people. We feel passionate about this,” he told me.

Still, the art and execution of “tishen” has been connected with more controversial subject matter of late. Many people here think these body doubles make are used in China’s court system. In fact, when it comes to justice, they say you can’t always believe what you see.

Take the case of a woman called Gu Kai Lai, who was convicted of murdering a British businessman Neil Heywood after a seven-hour hearing last August. The case attracted huge publicity, in part because she’s married to Bo Xilai – an important politician once tipped for the top. Yet the trial – and the murky details which surrounded it, have effectively killed off his career.

Millions of China’s internet users – or “netizens” – were fascinated in this case for another reason however. They wanted to know whether the Gu Kai Lai shown in court – was the real Gu Kai Lai. Snaps of Ms Gu taken in the months and years before the trial were compared online with the television pictures of the woman in the courtroom. Many were convinced that they couldn’t possibly be the same person. (Make your own mind up when you watch my report).

We met one such person – a teacher called Wang Zheng. Her theory went like this – the authorities were so worried about Bo Xilai’s popularity that they used a “replacement Gu Kai Lai” to ensure they got a verdict which discredited the entire family. Ms Wang told me, “the trail against Gu Kai Lai was used by the authorities as a way to get rid of Bo Xilai. They found a body double so they could control what she said.”

It’s difficult to judge whether these allegations are true – but they probably say more about the general atmosphere of mistrust that surrounds China’s justice system. There is a widespread perception here that the wealthy and the powerful can escape justice – and the lack of legal transparency only serves only to bolster this scepticism.

Take the case of Hu Bin for example – the son of a well-to-do merchant from the city of Hangzhou. He killed a man in his high-powered sports car in 2009. Local people were angered when he received what was considered to be a lenient three year sentence for his crime – and that sense of frustration was heightened when the man who appeared in court bore little resemblance to pictures of Hu Bin taken shortly after the accident.

More certainty surrounds the case of Shi Jianfeng – a man sentenced to life imprisonment last year for not paying highway tolls. Yet but Mr Shi later revealed that he was standing in for his brother – a truck driver by trade. A retrial was ordered – and two judges were apparently fired for failing to notice that they had convicted the wrong man. Mr Shi told a local television crew, “I went to prison for my brother. It’s as simple as that.”

Serving and former officials told us that the use of body replacements in China’s prisons is common. We met a former prison guard called Lao Li who told us that, “every prison has people serving time for crimes they didn’t do. It’s not just one or two cases either. This method has existed for a long time.”

I asked him why innocent people go to prison. He told me migrant labourers often volunteered; “they make more money taking someone’s place in prison – sometimes they make twice as much as their normal salary.”

Suggestions of “tishen” fuel suspicion in China’s justice system – and the feeling that a privileged few live by a different set of rules. They also produce the sort of unhappiness and sense of grievance that China’s new set of leaders (who took office in November) would do well to take note of.

Don’t forget, the country’s online citizens are on the lookout.

Follow @c4sparks on Twitter.