Lyndon B. Johnson's presidential portrait still stares out from the lobby of the imposing four-story U.S. embassy overlooking the Congo River here.

President Carter's should soon be hanging in its place, now that the United States and the People's Republic of the Congo have decided to "normalize" diplomatic relations suspended 12 years ago.

Back then, the roughing up of three American diplomats at Brazzaville airport prompted Washington to suspend deteriorating relations with this former French colony. Two years earlier Congo-Brazzaville had become one of the continent's first self-proclaimed Marxist states.

Now the Americans -- and any other Westerners with money to spend -- are welcome, but may choose to tread lightly. For the Congo is beginning to emerge from a still mysterious springtime week of violence which established something of a record in coup-prone Africa -- the incumbent president, his predecessor and the country's first and only cardinal were killed. Contrasts and Contradictions

SO FAR, the new leadership headed by President Joachim Yhombi-Opango has blamed "imperialism and its lackey," but foreign diplomats and most Congolese are convinced the violence was a tribally motivated internal power struggle.

Despite its denunciation of imperialists, the government is wooing the capitalist countries and insists that it is only "continuing" policies initiated by the murdered president, Maurien Ngouabi.

The new leadership has plastered walls with black-bordered portraits of Ngouabi, Brazzaville University has been renamed after him and the daily dose of Marxist vituperation against imperialists and their works has been stepped up on the "Voice of the Revolution," as Radio Brazzaville calls itself.

But, as one of the 8,000 French residents here said privately. "No one is fooled by all the left-wing propaganda --

Economic necessity -- rather than any ideological change of heart -- appears to have dictated the regime's overtures to the West. The economy is in shambles, thanks to a combination of blind application of the rigid Soviet model and a governmental spree. By Yhombi-Opango's own admission, almost all the 49 state-owned enterprises were badly managed and in the red. The government itself overspent on the basis of overoptimistic oil revenue projections and compounded its problem by hiring more civil servants rather than making economically productive investments.

In fact, only the steadfastness of French governmental financial and other aid helped the Congo weather the storm. The Marxist leadership had never left the French franc zone and, by this, ensured international financial and monetary respectability despite its radical rhetoric.

But such protection has its price, "congolese political will goes in one direction, economic reality pulls in another." an Eastern European diplomat said.

Indeed, Brazzaville is a schizophrenic society. Few Soviet bloc vehicles are visible and the approximately 400 Soviet advisers keep a generally low profile. But French and Japanese cars drive along roads still named for President Paul Doumer of France's Third Republic or for Albert the First, king of the Belgians.

The headquarters of the country's only party -- the Marxist Congolese Labor Party -- boasts giant portraits of Marx, Engels, Lenin, Che Guevara, Mao, Ho Chi Minh, Patrice Lumumba, Salvador Allende and now, Ngouabi.

Other billboards proclaim revolutionary Marxist slogans -- although one echoed President John F. Kennedy's inaugural exhortation reading. "Ask not what the people should do for you, but what you should do for the people."

Mindful of morality, the ruling military committee of the party has issued orders banning bikinis. But prostitutes follow prospective customers down the peeling halls of a seemingly instantly decreipt Soviet-built hotel.

The regime is also capable of arbitrary abuse, as Senegalese merchants discovered last month. With almost no warning, their humble shops facing major thoroughfares here were smashed by soldiers. The official reason was that the single-story buildings were not fancy enough for the capital.

This display of intra-African racism contrasts with the treatment reserved for South Africa's white rulers. The president denounces them as "Hitlerian fascists." But recently a French airline was authorized to resume, after a hiatus of years, using Brazzaville as a stopover on its Paris-Johannesburg run. What's known euphemistically as "Botswana meat" and South African fruit and vegetables are on sale. Mysterious, unmarked jet freighters fly in and out of Brazzaville under cover of darkness.

Such behavior led an embittered Congolese intellectual to complain. "The revolution stops outside the broadcast studios of the Voice of the Revolution." The Sidewalk Radio

THE NEW leadership remains nervous enough to maintain an 11 p.m. curfew, although soldiers have stopped patrolling Brazzaville streets with automatic weapons struck menacingly out of rolled-down car windows.

So incomplete was the Military Committee's explanation of Ngouabi's assassination on March 18 that it was interpreted abroad as the work of leftists determined to stop his opening to the West.

(By coincidence, West German ambassador Christoph Derix, who represents American interests here, had called at the president's office only hours before the killing, seeking a meeting with Ngouabi to deliver a Carter message urging renewed relations.)

But Radio Trottoir, the "sidewalk radio," as the gossip mill is called here, decided right away that Ngouabi was killed by fellow army officers from the north with whom he had ruled since 1968.

Its version, credited by foreign diplomats, has the virtue of recognizing the reality of tribal politics rather than supposed foreign policy or ideological motivations. The facts of political life are that the 1.4 million Congolese are dominated by army officers from the sparsely inhabited north. By throwing a few crumbs to the southerners, they rule over the more numerous and advanced people of the center.

Dogged by the economic mess and the attrition of years in power, Ngouabi, according to the sidewalk radio, was about to strike a deal with Alphonse Massemba-Debat, the first Marxist president, who had been ousted in 1968. The story was that Massemba-Debat, who was from the central Congo, was to have taken over the government while Nogouabi was to have settled for party leadership in what implied a diminution of northern power.

Some Congolese say Massemba-Debat and the long-suffering centrists were involved in a plot of their own. In any case, it now seems clear the northerners decided on a preemptive strike.

The only certainty is that Ngouabi was killed by gunfire during the siesta -- at 2:30 p.m. -- in the normally heavily guarded general staff headquarters which served as both his home and office.

The first official version said the deed was done by a four-man suicide squad led by a cashiered army captain named Barthelemy Bikididi who had long lived in exile in France. Later, another official communique said Ngouabi was gunned down by a member of the presidential guard in the captain's pay.

The captain conveniently disappeared and has not been found to this day. The member of the presidential guard was executed before he could do more than mouth the most pro forma confession.

That, too, was the fate of Massemba-Debat. He was arrested as the brains behind the assassination and executed by orders of a special revolutionary tribunal before dawn six days later. The Voice of the Revolution broadcast snippets of his alleged confession, but they failed to convince many Congolese.

As for Emile Cardinal Biayenda, he lost his life for peculiarly African reasons. Involved in the political bargaining, the cardinal had the misfortune to be the last visitor received by Ngouabi, the meeting ended barely an hour before the killing.

Government officials and church authorities agree that, in a society still strongly influenced by witch doctors and their fetishes, the visit of such a prestigious religious leader was interpreted as having deprived the president of the powerful protection which normally went with his office.

On the afternoon of March 23, four soldiers believed to be Ngouabi's relatives drove up to the cardinal's simple quarters opposite the French-built brick cathedral in a Land-Rover and demanded he accompany them to headquarters.

The cardinal's buried body was found early the next morning just outside the capital. He had been knifed to death. The Profit Motive

POLITICALLY, the biggest surprise was Yhombi-Opango's emergence as leader of the military committee, all but two of whose members are northerners. His leadership is officially explained by the fact that, at 38, he is the country's only colonel. But during the first weeks after the violence, the sidewalk radio bet on Defense Minister Denis Sassou-Nguesso, the 32-year-old, Algerian-trained radical darling of the Russians and Cubans, who is generally supposed to have instigated Ngouabi's demise.

Yhombi-Opango has stripped his rival of his vital Security Force command. The new president further sought to consolidate his still fragile power by dissolving the National Assembly, emasculating the party and making his signature mandatory on all orders.

The sidewalk radio reports that Sassou-Nguesso, who is guarded by Cuban bodyguards, has sought psychiatric help.

Yhombi-Opango has proven himself capable of publicly charging the local French business community of greed, then flying to Paris for an official visit and pleading with other French businessmen to invest in the Congo.

Many of these businessmen were old Congo hands who had amortized their businesses here long before their expropriation by the regime.

The president received a warm response in Paris. For the bottom line for these hard-headed men was not the jargon about advancing scientific socialism but rather the new president's straightforward admission that "almost all the 49 state enterprises are in the red."

His harshest words since taking power were in fact reserved for the Congolese labor unions on May Day. "We're dealing with state enterprises which were badly thought out, badly organized, overstaffed, unproductive and budget-devouring," he said. "Laziness and theft seem to be the rule," he added. "The advice of both foreign and national experts is practically never followed."

Such tough talk helps explain why the French are delighted with the new team and hope that the Congolese comrades will get down to making money for all concerned.

But even that may not prove a sufficient incentive to woo back home the hundreds of educated Congolese who went into exile rather than put up with the vagaries of the regime.