Secretary of State George P. Shultz introduced a new U.S. ambassador to El Salvador yesterday and announced that both the personnel and the policy are now in place to deal with strife-torn Central America.

Shultz' steady-on-course viewpoint was shared by President Reagan, who said as he left for the weekend at Camp David that the planned dispatch of new U.S. medical teams to El Salvador does not constitute an escalation of U.S. involvement.

At a hastily summoned news conference, Shultz revealed that veteran diplomat Thomas R. Pickering has been selected as the new U.S. envoy in San Salvador. Pickering, currently ambassador to Nigeria, is reported to have been the choice of Shultz and of dismayed career Foreign Service officials who sensed that Central America policy was being taken out of their hands and lodged in the White House.

As late as Sunday, according to official sources, retired Adm. Gerald E. Thomas, currently ambassador to Guyana, was the White House choice for the sensitive El Salvador post. But Shultz advanced the name of Pickering during Tuesday discussions in Williamsburg, and the career diplomat hastily flew here Wednesday from London, where he had gone for medical treatment.

Pickering got the final nod from Reagan at an unannounced White House meeting yesterday morning, sources said.

Pickering, 51, joined the Foreign Service in 1959 and has served as ambassador to Jordan, executive secretary of the State Department and assistant secretary for oceans and international environmental and scientific affairs.

Officials said Thomas, who was one of the Navy's senior black officers, is being given a consolation prize of the ambassadorship to Kenya, a much more important and populous country than Guyana.

White House and State Department sources said there is no plan to replace the U.S. ambassador to Guatemala, Frederick Chapin, at this time, despite reports to the contrary Wednesday night. A new U.S. ambassador to Costa Rica, Curtin Windsor Jr., was named by the White House five weeks ago.

Under persistent and skeptical questioning, Shultz insisted that the personnel shifts in Central American diplomacy, including the sudden replacement last week of Assistant Secretary of State Thomas O. Enders, are not a sign of a change in U.S. policy directions.

Citing Reagan's April 27 address on Central America to a joint session of Congress as an authoritative statement of U.S. objectives, Shultz said, "That policy has been in effect, remains in effect and is fully supported by all the people here with me."

Standing behind him in symbolic array as he spoke were Enders, Pickering and Langhorne A. Motley, U.S. ambassador to Brazil, who has been named as Enders' successor in the top Latin American policy job.

To the charge that U.S. policy in the region increasingly is being militarized, Shultz replied that "the militarization has been brought about by Soviet and Cuban to Nicaraguan military capabilities" applied through leftist guerrilla action in El Salvador and the export of revolution from Nicaragua. He said that any militarization from the U.S. side has been in response to this challenge.

Shultz paid tribute in elaborate fashion to Enders and Deane R. Hinton, the departing U.S. ambassador to El Salvador, and also praised their successors, Motley and Pickering, in warm terms. Enders is expected to be named ambassador to Spain, and Hinton to receive an as-yet-unspecified diplomatic post.

Reagan, in a brief question-and-answer session at the White House, confirmed that U.S. military doctors are being sent to El Salvador to treat military and civilian patients.

Asked how he would reply to those who see the military medical teams as a step-up in U.S. involvement in that country, Reagan said: "Well, if they say it, they will be as wrong as they've been on so many other things. Because, as I said, there is a real need for medical care down there, and in the civilian society as well."

He said the teams were being sent after consultation with Congress. The reaction of "key members of Congress" has been that of "understanding and general approval," according to a White House statement.

The U.S. team of 20 to 25 doctors, technicians and medics from various branches of the armed forces is being dispatched in response to a request from El Salvador, according to the White House. Spokesman Larry Speakes said a U.S. survey team to plan for the medical mission is now in El Salvador, and that an earlier survey team had made a report to Reagan.

Citing the serious need, which Speakes said had been recognized as "overwhelming" two or three months ago, another administration official said additional medical teams might be sent to El Salvador, perhaps from units now in Panama, if the situation seems to require it.

Also under consideration, Speakes said, is "a medical health services emergency package" to provide more aid to the civilian population.

The military medical personnel are not being counted against the self-imposed limit of 55 U.S. military advisers in El Salvador, the White House said.

At the Pentagon, officials showed reporters a series of color photographs of conditions at the only military hospital in El Salvador in an attempt to dramatize the need for American help.

A Salvadoran was shown arched in pain as he lay on a hospital bed. A doctor was cleaning the wound from an arm amputation. Pentagon officials said the doctor had put a block of wood in the patient's mouth for lack of painkillers.

Jerry M. Brown, director of international health affairs at the Defense Department, said Lt. Col. Edward Lynch of the U.S. military's Southern Command in Panama had taken the pictures for use by the U.S. Embassy in El Salvador.

Pentagon officials said it was the ambassador who had launched the plan to send an American military team of physicians, nurses, corpsmen and administrators to the military hospital in San Salvador.

Col. Jay C. Bisgard, deputy assistant secretary for health affairs at the Pentagon, said there is no plan to send the American medical specialists out into the field where they might get shot.

He added that the specialists probably would not remain at San Salvador's 100-bed military hospital, which treats both soldiers and civilians and has been overwhelmed with war casualties, for more than six months.

Meanwhile, there were these other developments regarding Central America:

Members of the House Foreign Affairs Committee put off to next week efforts to reach bipartisan agreement limiting undercover U.S. aid to rebel forces fighting the Sandinista government in Nicaragua. Rep. Lee H. Hamilton (D-Ind.) is circulating a proposal that would allow U.S. aid "solely for the purpose of intercepting ongoing arms shipments across international borders."

Richard Stone, the newly installed special envoy for Central America, departed on a 12-day mission to the area. Included in his itinerary is a June 10 visit to Nicaragua, which will provide the opportunity for a rare, high-level diplomatic contact between Washington and Managua.

Shultz said Grenada Prime Minister Maurice Bishop, who had been asking for an appointment with Reagan during a current visit to Washington, had been granted a meeting with J. William Middendorf II, U.S. ambassador to the Organization of American States, but that Bishop did not appear.

In a statement last night, Grenada's ambassador to the OAS, Dessima Williams, said his country regarded the offer of a meeting between Bishop and Middendorf as "an affront to the integrity of Grenada . . . quite inappropriate as well as a breach of diplomatic norm."

The statement added that Bishop continues to want an appointment with Reagan.