The newspaper said the photo was taken on 15 December 2020 - while Covid restrictions were in place - during a No 10 “virtual” Christmas quiz.
But it is not one of the parties currently under police investigation.
Labour MPs called on Mr Johnson to refer the gathering to the Met after the photo surfaced, but the PM said the force had already examined the event as part of their investigations.
The prime minister’s spokesman later insisted to reporters that the gathering in the picture was a “virtual quiz”.
But the police then released a statement, saying: "The [Met] previously assessed this event and determined that on the basis of the evidence available at that time, it did not meet the threshold for criminal investigation.
If you look carefully at the latest photograph you will notice that everyone in it is wearing the same clothes as in a previous photo. This new photo is taken from a different angle that’s all. This tells me that it’s the same event as one already being investigated and NOT a new one.
The Christmas quiz is not one of the dozen No 10 gatherings under investigation by police.
This is despite a smaller festive quiz held in the Cabinet Office just two days later being looked at.
One Covid laws expert said that Scotland Yard should now add the Downing Street Christmas quiz to the list of events it is probing.
Barrister Adam Wagner told The Mirror: "The previous photograph of the Prime Minister showed what could have been a meeting briefly interrupted by a Zoom call.
"However, the new photograph shows four people with alcohol and food, as well as a Christmas hat and tinsel, which suggests a Christmas party.
Dame Cressida said she had been left with “no choice” after London Mayor Sadiq Khan made it clear to her he had no confidence in her leadership.
Last week, the police watchdog found “disgraceful” misogyny, discrimination and sex harassment among some Met PCs.
Dame Cressida, the first woman to lead the biggest UK police force, also faced criticism over the Sarah Everard case.
Her conduct was referenced in an independent report that accused the force of institutional corruption over the unsolved murder of Daniel Morgan, and before she became commissioner she was in charge of the operation that led to the fatal shooting of Jean Charles de Menezes.
How that will affect the investigation is uncertain but, apparently, Dick is staying on for a while so the report may well be concluded before her departure.
Prime Minister Boris Johnson said she had “served her country with great dedication and distinction over many decades”, thanking her for “protecting the public and making our streets safer”.
Home Secretary Priti Patel - who is responsible for the hiring and firing of Met commissioners - also put out a positive statement, saying Dame Cressida had “undertaken her duties with a steadfast dedication to protecting our capital city and its people”.
And Health Secretary Sajid Javid thanked her for her public service, calling the role “one of the most difficult jobs in the country”.
For the Liberal Democrats, the focus was on Dame Cressida’s successor and who should - or should not - be involved in the process.
Party leader Sir Ed Davey said the prime minister “must have no role in choosing [her] successor” while the force is investigating rule-breaking parties in No 10 during the pandemic.
The decision on who will take over falls to Ms Patel. But Lib Dem MP Munira Wilson called on her to give a “cast-iron guarantee” that she won’t involve the PM at any stage.
The force, which was heavily criticised for an apparent hesitation to launch a probe into the alleged parties, said the investigation continues as normal and remains under the control of Commander Catherine Roper.
Exactly this. The image has been Photoshopped - slightly off vertical champagne bottle and no drinking glasses?
Another Daily Mirror stitch up on par with the alleged photos of British soldiers torturing civilians claimed by Piers Morgan.
Those three are not the only ones who have lost confidence with Cressida Dick. Her tenure as the Met police chief has been a disaster, from the shooting of Jean Charles de Menezes in 2005 which they tried to cover up and then changed emerging facts to cover their narrative, to the rampant knife crime in the City of London, preferring to ignore real crime and concentrate on appeasing protestors. She only got her position because she ticked all the boxes rising through the ranks, not from any competence on her part.
