They are sometimes known as "handshake photos." Mr. or Ms. Local Citizen poses proudly shaking the hand of Mr. President or Mrs. First Lady.

Dozens are snapped as the presient or the first lady meets another dozen people at another dozen receptions. And occasionally, among the average citizens smiling are some not-so-average citizens also smiling and shaking hands with the president or the first lady.

Like then vice president Hubert Humphrey smiling with the Rev. Sun Myung Moon, the Korean evangelist.

Or former president Gerald Ford standing with a cheerful group of people that included Korean businessman Tongsun Park, once under investigation for bribing congressmen.

Or most recently, Rosalynn Carter smiling and shaking hands with John Gacy, former Democratic precinct captain in Illinois, now suspected of murdering 32 young men after having sexual relations with them.

The Gacy photo was autographed: "To John Gacy, Best Wishes, Rosalynn Carter."

"There is no way we can know everyone who is shaking hands with the president or with Mrs. Carter or having their pictures taken with the president," said one member of the president's advance office.

Advance staffers often guide the president or his family members at different events, sometimes setting up pictures. "You don't know who everybody is and you don't know what's going to happen to them later in life. Whatever else is said about advance people, we are not clairvoyant."

Still, it's enough to mak any press secretary break out in a cold sweat.

"If you are a president or a president's wife," said George Reedy, one-time press secretary to president Lyndon Johnson, "it's almost impossible to avoid having your picture taken with someone you wish you had not had your picture taken with."

Everyone wants to have his picture taken with the president. And ever since the days of Franklin Roosevelt, presidents have seen the political potential.

When former president Johnson once stepped from a car to shake hands at an event, the governor of the state truned to him and asked Johnson why he bothered. "Do you know how important this is?" Johnson was heard to say. "This man will go home and tell his wife and children he shook hands with the president of the United States."

And if there's a picture, he'll take the picture home and pin it on his wall for all his friends to see.

So rather than hang back from crowds that might contain a future criminal or two, presidents tend to plunge right in. And not everyone has been checked out by the Secret Service.

"The president might say to someone after a speech, 'Hey come up here,' and they'll shake hands and take pictures," said Secret Service spokesman Ken Lynch. "Or the president can hop out of a motorcade and start shaking hands."

In June of 1976, as presidential candidate Jimmy Carte wove his way through a crowd at a party, Suzi Park Thomson hopped up to shake Carter's hand just in time for the camera to catch the handshake and Carter's broad smile. At the time Suzi Park Thomson was being linked to some congressmen under investigation in the Korean bribe scandal.

Former president Harry Truman, once seated at a piano at a party, as he often was, found himself snapped with the young and leggy Lauren Bacall who had hopped up on the piano at the right moment for a photographer. The president was not pleased, she relates in her autobiography.

Still, cameras go clicking away. "It's almost like live-action camera, you're taking so many photographs," said one White House photographer. Whole reception lines of people meeting the president or his family can be snapped, one shot after another.

At least in that situation, when the line is full of guests invited to a White House function, the outcome is less risky.

On the road, despite all Secret Service precautions, everything changes.

"When you go out to another location," says one former White House aide, "you're at the mercy of the local politicos. When I went out to one location with Carter, the local organization had already lined up about 30 people for him to run through."

And often that is all to the good. For instance, in Chicago, where the photo of Rosalynn Carter and John Gacy was snapped, another political observer noted, "You're dealing with a strong Democratic organization and the last thing you want to do is send in a strong advance man who's going to muscle people around."

In October 1976, then-presidential candidate Jimmy Carter was hosted at a waterfront rally in New York City by Anthony M. Scotto, a prominent leader of the International Longshoremen's Association. It was one of Carter's final pre-election campaign appearances. Pictures of course were taken. Recently, Scotto was indicted by a federal grand jury on charges of racketeering, mail fraud, income-tax evasion and taking illegal payments from waterfront businessmen.

Another time in 1965, vice president Hubert Humphrey was speaking at a dinner in Florida when someone invited the vice president up to his suite afterward. According to one former Humphrey aide, when the vice president reached the suite, he found a group of men waiting. Pictures were taken. Within minutes of leaving the suite, Secret Service agents told Humphrey they had reason to believe the men were in some way connected with organized crime. The Secret Service returned to the suite and demanded the film. The men refused, but after some dispute, the Secret Service came up with the film. No one knows what happened to it after that. Or for that matter, who was in it, the ex-staffer said.

And then there are times when savvy aides can head off embarrassing moments. In the '76 primary Gerald Ford arrived for pictures at Disney World in Orlando, Fla. A 4-foot-high Mickey Mouse danced around in view. An aide figured Ford would make a beeline for the mouse when he arrived. He knew the picture wouldn't work. Ford arrived. He made a beeline for the mouse. The aide went running after him, elbowing him away, stopping him just in time. "We'd seen a picture of Hirohito with the mouse," said the aide, "and it didn't look that good." CAPTION: Picture, Rosalynn Carter posed with John Gacy and then autographed this photograph after a reception in Chicago on May 6, 1978; Copyright (c) 1979, The Chicago Sun-Times.