Trump, who attended the day one of the summit pitched the idea of discarding all tariffs and trade barriers between the USA and its G-7 allies.
Canadian Prime Minister Justin Trudeau said his country "will not be pushed around on trade matters." and called USA tariffs on Canadian steel and aluminum "insulting".
Her comments came amid an escalating fallout from a contentious meeting between Trump and other world leaders at the Group of Seven (G-7) Summit in Canada over the weekend.
"PM Justin Trudeau of Canada acted so meek and mild during our @G7 meetings only to give a news conference after I left saying that, "US Tariffs were kind of insulting" and he "will not be pushed around".
"We need Russian Federation for disarmament talks" and for discussions about both Ukraine and Syria, Merkel said.
Like virtually all German media, Berlin's daily Tagesspiegel ran the picture prominently and said Trump's caustic tweets upending the summit conclusions had "shaken the West".
The BBC parsed the picture by identifying all nine individuals in it, including British Prime Minister Theresa May, who is barely visible, and Larry Kudlow, head of Trump's National Economic Council as well as several Japanese officials and USA national security adviser John Bolton.
Tensions between the USA and the European Union saw the G7 summit end in chaos after Mr Trump refused to sign the joint statement of world leaders. Speaking on the condition of anonymity a Canadian official told national broadcaster CBC that not only had Trudeau made these remarks prior to the meeting, on USA television last week, he had also informed Trump of his position before he went on air.
"As Canadians, we are polite, we're reasonable, but also we will not be pushed around", Trudeau said.
US National Security Advisor John Bolton, who stands next to Trump in the picture, tweeted it during the summit to tout the America First message.
Shortly after that statement was published online, Trump complained about comments made by Trudeau and revoked his support. Asked by reporters whether he would relent on tariffs, Trump looked at Merkel and said: "I don't know".