The artwork by well known Hull graffiti artist Preston Fox - otherwise known as Preg - has gone up on boarding on Finkle Street, off the A63, in the city.
It shows a young boy with a rose standing on a box to get a better look at the image of Mr Alder, who choked to death while handcuffed on the floor of a police station in Hull in 1998.
An inquest jury found the 37-year-old former soldier had been unlawfully killed in 2000. His sister Janet has continued to battle for answers ever since.
In a horrific twist his body came to be discovered in a mortuary 11 years after supposedly being buried, a Nigerian woman Grace Kamara having been buried in his grave instead.
A 17-month police investigation into the blunders could not determine “with any accuracy” how the bodies came to be exchanged, or who was involved.
Above the image is a magpie - one for sorrow - and the portrait is entitled Justice for Christopher Alder.
Already a bunch of flowers has been left in front of the artwork and there are calls for it to be protected.
Ms Alder, who lives in Halifax, is still trying to discover why she was the subject of a "spying" operation by police during the inquest.
Last year a panel concluded there was no case to answer against two officers accused of gross misconduct over the unauthorised surveillance on July 28 2000.
Ms Alder said: "I absolutely love it. I think it is excellent that someone has gone out and remembered not just one injustice but two - the killing of him and the hiding of his body.
"There has been no resolution of anything. It is 21 years this year.
"Of course you get the negative ones. but I have learned over the years not to let it bother me. They only know a fraction.
"There was a fantastic jury at the inquest who were able to see what happened and they must have passed the story onto people."
She said she would have liked to have left some kind of monument to her brother in Hull - but feared it would not be respected.
She added: "I am really happy about this."
Artist Preg said he was still learning about the Christopher Alder story - and it was a "real eye opener."
He said: "It beggars belief what happened. It has been a complete fiasco. Everyone seems to have let him down."
The many comments on social media recalled the tragedy, one friend describing Mr Alder as "a true gentleman and a dear friend."
Another said: "(He) was my friends uncle always gave us pocket money as kids always a smile for everyone."
Another said: "Rest in peace Christopher. Such a tragedy and horrific treatment by the police . Plus the body mix up. Absolute farce . May we always remember him and not the awful circumstances.he deserved respect and this was not given whatsoever."