Answer: NYC Protests

The anti-nuclear demonstration in New York on June 12, 1982, is estimated to have been from 400,000 to one million. (Common Dreams: Common Dreams). Pictures of the protest can be found here: War Resisters and here Culture of Peace.

From the New York Times:
"As many as 700,000 people took part in the events, which were peaceful.‿
ANTINUCLEAR PROTEST COST THE CITY $1.8 MILLION by MICHAEL GOODWIN, July 25, 1982, Sunday, Late City Final Edition, Section 1; Part 1; Page 30, Column 1; Metropolitan Desk--from Lexis-Nexis, Aug 29,2004

Patrick J. Murphy, the Police Department's chief of operations, said at 2:30 P.M. that the crowd on the Great Lawn of Central Park - even without those who overflowed onto Fifth Avenue and Central Park West - had reached 500,000. Tens of thousands more were still on the parade route from the United Nations and Dag Hammarskjold Plazas at 47th Street and First Avenue, waiting for places in the park. Late in the afternoon, Alice T. McGillion, a Police Department spokesman, said there might be as many as 600,000 or 700,000 people around the park. ''There's so much movement it's hard to be certain,'' she said. Other estimates of the size of the crowd were even higher. As the marchers entered Central Park, Michael Siegel, a park ranger, addressed them through a megaphone. ''You have made the world's largest peace rally ever,'' he said. ''We estimate three-quarters of a million people.''
THRONGS FILL MANHATTAN TO PROTEST NUCLEAR WEAPONS by PAUL L. MONTGOMERY, June 13, 1982, Sunday, Late City Final Edition, Section 1; Part 1; Page 1, Column 2; Foreign Desk--from Lexis-Nexis, Aug 29,2004

You might find more pictures on the following videos, they each list “nuclear protest‿ in descriptions of their content:

1982, history of the 80's
ABC News.; Bolthead Communications.; MPI Home Video, 1991

People's century, 1900-1999. Fallout
David Espar; et al, 1999.
WGBH Boston Video

Simon and Garfunkel’s Concert in Central Park drew more than 500,000 people on September 19, 1981, according to Online Encyclopedia Information Genius).

The best online picture of the crowd we found was on the cover of the video of the concert (yahoo). You can get a copy of the concert at a library or video store. There are also biographies of Simon and Garfunkel that include illustrations that might have crowd pictures of the concert: Simon & Garfunkel: the Biography, by Victoria Kingston (1998); Simon and Garfunkel: Old Friends, by Joe Morella (1991); Simon and Garfunkel, by Robert Matthew-Walker (1984).