On the outside the box lunch for yesterday's novel White House staff picnic was adorned with a green and white sketch of the White House with the Pennsylvania Avenue-facing North Portico stretched into a large, toothy grin.

The inspiration for the illustration, President Jimmy Carter, joined a couple of hundred staff people on the South Lawn for a few minutes, chatted with the Jazz Minors, a seven-piece Dixieland-style band from Oregon, stopped to speak with the Oregon congressmen, Mark Hatfield and James Weaver, and then retreated to his own lunch with Walter Mondale. The President arrived with Rosalynn Carter, who was given a stuffed tiger for Amy by the musicians, and with George Peppard, the actor, who had been visiting the White House yesterday morning.

After listening to the musicians for a few minutes, with his foot up on the piano stool, Carter told the banjo player, "This is a rare occasion for us to have the people who work here eating on the lawn." Only a few brave staffers overlooked the threating skies and sat on the lawn. Most of the 400 who paid $3 for lunch sat on folding chairs, including Frank Moore, congressional liasion; James King, special assistant for personnel; Timothy Kraft, special assistant for appointments; Esther Peterson, special assistant for consumer affairs; and Richard Harden, special assistant for Budget and Organization.

Over a lunch with the President last month, Gretchen Poston, the First Lady's social secretary, proposed the picnic. The White House had called the gathering a "Summer in the Parks" affair which confused the press corps, who expected some summertime programs the National Park Service sponsors under that name to be discussed.