A Microsoft spokesperson tells me that "accidental human error" is to blame for missing images of "tank man" on its Bing search results.
"We are actively working to resolve this."
The incident comes on the 32nd anniversary of the Tiananmen Square Massacre.
It's possible that the English term "tank man" wasn't censored on Bing image search in China before, but it is now. Over there the Tiananmen massacre is usually referred to as the June 4 incident, so it's usually the characters 六四 (6 and 4) that are censored in search results. Because Bing isn't a very popular website in China, it might be that "tank man" slipped through until now.