A young Bill Russell chatting with a New York player - at Fenway Park
The Boston Public Library has posted a ton of Leslie Jones basketball photos, including a number of Celtics photos, such as this one of Bill Russell at Fenway in September, 1962 and this one of Bob Cousy with the ball in a 1950s game at the Garden against the Knicks:
Here's Red Auerbach with some of his players in 1954 (can anybody ID them?):
Photos posted under this Creative Commons license.