The photograph was posted to McMaster's Facebook page in December 2010 but had been taken two years earlier.
The source said the use of the image raised questions over father-of-four McMaster's judgement.
He added: "Whatever way you look at it, it is a disgrace that an armed response police officer believes this is an appropriate image for a public audience.
"Putting something like this on a public forum absolutely beggars belief. McMaster should be ashamed of himself."
Armed response officer McMaster also has a photograph on his page from March 2013 of himself aiming a hunting rifle.
He also has a Mr Men-style cartoon of a muscular police officer described in the picture as "Mr Elite ARV/TSG officer".
The abbreviation ARV is short for Armed Response Vehicle, while TSG describes Territorial Support Group - the Metropolitan Police's riot squad.
Police chiefs have said McMaster will be given "appropriate advice" about his use of social media.
Chief Superintendent Elaine Ferguson said: "The first picture was taken on a holiday in New Zealand in 2008 on a visit to police station in a safe, controlled and supervised environment.
"The second picture was taken in March 2013 with a legally-held weapon while the officer was off-duty.
"Officers and staff are reminded of their personal responsibility when using social media in a private capacity.
"We will continue to look into this matter and provide the officer with the appropriate advice."
- Daily Mail