Mr Ross was one of the Tory MPs who rebelled against Boris Johnson in the wake of partygate at the start of the year.
The SNP has called on the leader of the Scottish Conservatives to resubmit a letter of no confidence in the Prime Minister.
However, Mr Ross, who is also an MSP in the Scottish Parliament, made a U-turn shortly afterwards, instead insisting there was a need for stability at the top of Government during the Russian invasion of Ukraine.
The Scottish Tory leader said on Monday that pictures released by ITV News, which showed the Prime Minster holding a drink while standing behind a table covered with bottles of wine and food on November 13 2020, would make people “very angry” and called for an explanation.
Mr Johnson was not fined for the event shown in the pictures – a leaving party for former head of communications Lee Cain during the second lockdown.
The SNP’s deputy Westminster leader, Kirsten Oswald, said Mr Ross is “weak and unprincipled” in his actions over the matter.
She said: “The photo evidence has destroyed Boris Johnson’s excuses and exposed his lies. It is now beyond doubt that the Prime Minister was partying during lockdown, breaking his own laws, and repeatedly lying to the public.
“Douglas Ross doesn’t need any more explanations. He needs to grow a backbone and resubmit his letter of no confidence without further delay.
“By constantly flip-flopping over Boris Johnson’s future, the Scottish Tory leader has blown any remaining credibility – and shown himself to be a weak and unprincipled, lame duck leader.
“The Scottish Tories have run out of excuses. The longer they leave Boris Johnson in office the more damage they will do to public trust in this out-of-touch UK Government.
“Tory MSPs should speak out like they overwhelmingly did in January. It is time for them to do the decent thing and speak out again.”
Conservative MSP Alexander Stewart told ITV’s Representing Border that he was “still disappointed, dismayed and disgusted” at the Prime Minister.
“I haven’t changed my views. That’s my view at the very start of this process, and I still believe that,” he said.
His fellow Tory MSP, Stephen Kerr, said he was “disappointed and angry” at the picture’s revelation.
But when asked by Representing Border if Mr Ross should resubmit a letter of no confidence, Mr Kerr replied: “That’s a matter for Douglas.”