Breivik, 32, is seen dressed in a police uniform with a gun as he walks away from a car after placing a bomb in Oslo.
Police confirmed the photo published on the ABC Nyheter website was authentic.
The car bomb killed eight people. Mr Breivik then shot dead 69 people at a youth camp on the island of Utoeya.
The photo of Mr Breivik came from monitoring cameras at government headquarters six minutes before the car bomb exploded, ABC Nyheter reports.
It said that no-one in the building raised alarm.
Oslo police district spokesman Roar Hansen told the Associated Press news agency that the image had not been released by police, but was part of a confidential police report submitted to lawyers assisting relatives and survivors of the 22 July attack.
Mr Breivik has admitted to the killings but denies criminal responsibility.
He has said that the massacre was “necessary” to save Norway and Europe from Muslims and multiculturalism.
Like most nutters he is convinced he is right. It is just tragic that sometimes these sick bastards have access to weapons – lots of weapons. But hey, if he was in America he would just be exercising his constitutional right to shoot people.