Last month, in an article on the U.S.-Soviet arms talks, Time magazine used identical pictures of national security affairs adviser Robert C. McFarlane. The caption under one identified him by name, while under the other he was described as an unnamed "senior administration official."

Six days later, a front-page story in The Washington Post said President Reagan planned to seek a budget compromise with Senate Republican leaders, attributing this to the ubiquitous "senior administration official." But a bracketed paragraph said: "This official later was identified by United Press International as Office of Management and Budget Director David A. Stockman."

These are signs of the times, currents in a growing stream eroding the hallowed but abused institution of the Washington background briefing. At a time when many Americans distrust government and the news media, what is happening is worthy of attention.

Essentially, the backgrounder is an undeclared conspiracy of adversaries in which officials disseminate and journalists receive information cloaked in anonymity. Most reporters accept backgrounders as a necessary evil rooted in foreign policy sensitivities, although some think backgrounders are a disease that broke out long ago at the State Department and has reached epidemic proportions. Whatever we think of backgrounders, it is clear that their ground rules increasingly are being honored in the breach.

ABC News, for instance, routinely shows pictures of McFarlane while presenting the views of that unnamed "senior official." Twice in recent months, The New York Times has identified McFarlane as the source of major briefings, once after he gave an on-the-record interview for television that appeared to contradict some of what he said on background in the briefing room.

None of this is original. The Post used the same techniques employed by Time on McFarlane to identify "senior officials" Henry A. Kissinger in the Nixon administration and Cyrus R. Vance in the Carter administration. Such actions, once controversial, have become commonplace. President Reagan might pause in his fulminations against unauthorized "leaks" to ask why.

The president would learn, if he cared, that backgrounders in his administration frequently are of little value to anyone. Usually they are flotation devices that give overcautious officials a chance to put out the party line without being held accountable for what they say.

Even when backgrounders are helpful, they frequently are unnecessary. Why should a newspaper reader receive an official's anonymous defense of the Strategic Defense Initiative when the same official is going to cover the same ground on television the next day? Why should Stockman define budget strategy on background and then insult a myriad of constituencies on the record?

Occasionally, backgrounders have been helpful and necessary. In 1981, when the U.S. and Japan reached an accommodation limiting imports of Japanese cars, U.S. officials provided a frank and detailed account of the talks. They did so on background, because a key element of the pact was the fiction that the Japanese were engaged in "voluntary restraints."

But in this case, readers and viewers received more information and context because of the background rules. Backgrounders also were helpful on maneuverings that led to withdrawal of U.S. troops from Lebanon, achievement of the Social Security compromise and White House staff members' role in getting rid of some embarrassing political appointees. Typically, Reagan was denouncing "leaks" and defending his appointees while subordinates were going out to get the rope.

More often, background briefings have become excuses for carelessness and unaccountability by government and the media. The result has been a growing contempt that inevitably results in compromised backgrounders and threatens their legitimate use.

Reporters who attend backgrounders should keep their word and observe the ground rules. But officials should realize that if every utterance is deemed worthy of a background briefing, the chances are that backgrounders won't be worth anything at all.

Reaganism of the Week: Defending oil exploration in the Santa Barbara channel in an interview with the Santa Barbara News-Press last Sunday, the president told of a "lovely old lady" who had complained because she could see an oil derrick from her home. "When they evidently didn't find oil, they left," Reagan said. "Then she said, 'I miss the lights at night.' "