In a year with so many world-shaking moments, our strongest visual stories covered impeachment, outbreak, caucuses, primaries, donations, delegates, shutdown, jobs, deaths, coughing, hospitals, testing, George Floyd, protests, tear gas, dreams, nursing homes, policing, the election, stimulus, symptoms, Beirut, power, the Supreme Court, ballots, wildfires, Trump’s taxes, turnout, swing states, masks, results, A.I., voter shifts, I.C.U.s, pollution and vaccines.