I looked it up, and here, in more detail, is a description of what happened.
"Karsh asked Churchill to remove the cigar in his mouth, but Churchill refused. Karsh walked up to Churchill supposedly to get a light level and casually pulled the signature cigar from the lips of Churchill and walked back toward his camera. As he walked he clicked his camera remote, capturing the ‘determined’ look on Churchill’s face, which was in fact a reflection of his indignantcy. Karsh recounted: “I stepped toward him and without premeditation, but ever so respectfully, I said, ‘Forgive me, Sir’ and plucked the cigar out of his mouth. By the time I got back to my camera, he looked so belligerent he could have devoured me. It was at that instant I took the photograph. The silence was deafening. Then Mr Churchill, smiling benignly, said, ‘You may take another one.’ He walked toward me, shook my hand and said, ‘You can even make a roaring lion stand still to be photographed.'”