"The TIME of Our Lives," which opens Wednesday at the National Portrait Gallery, is a zippy little show. What one notices at once is its range, its wit, its flash.

National portrait galleries are rather somber places, and ours is no exception. The figures on the walls look as if they knew they were posing for posterity. They tend to think deep thoughts in throne-like chairs, and their clothes are often black.These covers from Time magazine are not like that at all.

Beneath her rhinestones and white feathers, Cher, photographed by Avedon, has almost nothing on. Robert Rauschenberg is laughing, and posing among turtles, in his collage selfportrait. Beverly Sills has dressed up to look like Queen Elizabeth I. Slugger Reggie Jackson, not yet a Yankee hero, still an Oakland A, swings his might bat.

"Since ancient days, said Gallery director Marvin Sadik, "people have collected portraits of their emperors, their ancestors and heroes - but they very rarely ordered pictures of their cooks."

One of Julia Child hangs in the show, which presented the director with "yet another worry." The TV star herself was expected as a guest at last night's formal dinner. "What are we to do if the food's not good enough?

The editors of Time, who have given the Gallery all the cover pictures available from the past 20 years, were not considering posterity when these portraits were commissioned. It is refreshingly apparent that they were thinking of the week.

Time covers were not made to hang in bureaucratic offices, ancestral halls or board rooms. They reflect the competition of drugstore racks and newsstands. They were made to grab the eye.

And to entertain. Gazing from the walls are the presidents, their wives, Nabokov, Walter Cronkite and Martin Luther King Jr. These are just the sort of faces, respectable and familier, that one expects to find in a portrait gallery. But here they are accompanied by an imposter (Cifford Irving); wealthy welrdos (Howard Hughes); boxers (Cassius Marcellus Clay, who looked slim and sweet and innocent in 1963); malefactors (Caryl Chessman, Lt. William Calley); comedians (Lily Tomlin, Woody Allen) and Huge Hefner.

The Hefner, an amusing wooden statue with a Pentoon-shaped head, was made in 1967 by artish Marisol. The present, we all know, is not the heyday of fine protraiture. Art is permitted to shock or Laffle, but portraits, to succeed, are supposed to catch a likeness. Time, apparently for reasons of prestige, has commissioned portraits from many famous artists - Roy Lichtenstein, James Wyeth and even Elmyr de Hory, the famous (or infamous) forger. Their reputations do not rest on their pictures in this show, most of which are drags.

Most Time covers show a face accompained by props - Childs' by a fish in aspic, Chessman's by a gas chamber, Jimmy Hoffa's by a truck. It is a formula, but it isn't easy. James Wyeth's Jimmy Carter is awkward, lifeless, dull. Many of the finest covers in this show were done by lessthan-famous artists who had that formula down pat.

Boris Chaliapin, for example, portrayed Richard Nixon in 1959, and John Kennedy one year later. He also portrayed Jackie Kennedy, Althea Gibson, Arthur Schlesinger and coach Vince Lombardi of the Great Green Packers. Clearly, he is a pro. There are 13 Chaliapins hanging in this show.

Other cover regulars are also firstrate: Bernald Safran (Chessman, Barry Goldwater); Robert Vickrey (William Fulbright, Henry Luca); and James Chapin (James B. Conant, Thurgood Marshall).

Seen on the newsstand, many Time covers look like photographs or straightforward paintings. Here one notices, however, that many are collages. Judge Sirica's robe is real fabric, as is John Dean's suit. Marisol's stone sculpture of Nixon and Henry Kissinger Provides the former Secretary of State will one glistening glass eye.

The installation, like the art, is frequently amusing. There is a wall of Watergate, and another given to contemporary showbiz (it begins with Lily Tomlin and Merle Haggard, and ends with Cher, Woody Allen, Rostropovich). Wandering through the show, one can see Jimmy Carter age (he first appeared on Time in 1971), and Richard Nixon decline (there are three Nixons here, the first from 1959, the last from 1973), and as the show progresses, the grayness of the '50s fades and the '60s start to flash.

The labels on the wall all are condensations of Time cover stories.

Though Time has been around since 1923, its editors at first did nothing to preserve its cover art. Some portraits were discarded; some were taken home by staffers on the magazine; some were given to their subjects. A number of those saved have been abused. Some have been glued down to board, others have bleached by sunlight some were beaten up in the black of drawers. Time gave 900 pieces of cover art to the Portrait Gallery. Half of those show foreign leaders, and they will go on tour. Approximately 100 were chosen for this show, which will run through August at the Gallery, 8th and F Streets NW.