“It was a pretty quiet moment,” Jacob Sousa said this week from his job at the L.A. Louver gallery in the Venice neighborhood of Los Angeles, where David Hockney’s solo exhibition “Something New in Painting (and Photography) [and even Printing] ... Continued” is on view.

“There were only a couple of gallery visitors, and one of my colleagues said, ‘Joni Mitchell is here,’ so I grabbed my camera and waited for her on the second floor.”

It was there that Ms. Mitchell spotted Mr. Hockney, an acquaintance, and where Mr. Sousa asked the two if he could capture the moment.

“She smiled and stepped in front of the painting and took David by the hand,” the photographer said of a gentle image that, posted on Valentine’s Day to the gallery’s Instagram account, quickly went on to garner more than two million impressions and to be liked and reposted and commented upon by people as disparate as Josh Groban, Lana Del Rey and members of the British Parliament.