The photo, snapped by the Press Association news agency and published in several newspapers, shows a clipboard held by a uniformed officer Friday outside the Ecuadorian embassy where Assange has been holed up since June.
It goes on to suggest possible ways in which he could exit the building, such as in a diplomatic bag or vehicle.
The document also appears to warn of the possibilities of "distraction," perhaps by supporters of Assange who have previously rallied outside the central London embassy building.
UK police have previously made clear that they intend to arrest Assange if he leaves the embassy, where he is shielded by its diplomatic protection.
But the carelessness of an officer in apparently exposing details of the force's plan will likely cause red faces.
Asked about the photograph, a spokeswoman for the Metropolitan Police told CNN "the document is not related to the Julian Assange case."
The apparent slip-up is the latest twist in the Assange case, which has brought unlikely drama to the posh Knightsbridge neighborhood since he sought refuge in the embassy to avoid extradition to Sweden to face questioning over sex crime allegations.
This month, Ecuador officially offered Assange asylum in the South American country, but the British say they will not give him safe passage out of the embassy.
The Foreign Office says Britain has a legal obligation to hand him over to Sweden, after Assange's legal efforts to avoid extradition were rejected by British courts up to the Supreme Court.
The dispute gathered heat when the British Foreign Office, in a letter to Ecuadorian officials, cited a little known law that could temporarily suspend the embassy's diplomatic protection and allow authorities to enter and arrest Assange. Ecuador's resident has dismissed any steps in that direction as "suicidal."
Assange's lawyer, Baltasar Garzon, said his client was willing to answer Swedish prosecutors' questions, but only if he was given certain guarantees.
Assange, an Australian, and his supporters claim a U.S. grand jury has been empaneled to consider charges against him.
He says the allegations against him in Sweden are politically motivated, and claims to fear Sweden will transfer him to the United States, where he claims he could face the death penalty for the work of WikiLeaks.
Sweden angrily rejected that allegation last week.