Once there was heroin chic. Its catwalk disciples had shadowed eyes, chalk faces and cadaverous cheeks. If they were high on anything, it was a cocktail of champagne, Marlboro Lights and irony. Now there is only heroin shock. The pictures of Rachel Whitear, killed at 21 with a syringe in her hand, are distressing both as an illustration of the ravages of death and for their contrasting tableau of a deeply normal life.  In the tidiest of student rooms, a girl with shiny hair and a striped dress crouches in a pose of pain or supplication. She had tried to get off drugs. Her counsellor saw her as a success story. Yet she died despite her resolve and the bright future that beckoned before she dropped out of Bath University. Her mission, ordained by her parents, is that she should now spare other children a fate from which she could not absolve herself.   Will her cautionary tale, a modern take on Belloc or Hoffman, have a lasting impact? Current wisdom says yes. Last week the British Medical Association urged the Government to revive the shock Aids campaigns of the Eighties. In their absence, sexually transmitted diseases have risen alarmingly. Complacency kills in a country where fortunes are spent to promote wrinkle creams and cars, and almost nothing on saving or enhancing lives.   But the advertising of risk is inexact. What message, exactly, should teenagers absorb? Rachel's mottled body, complete with 'Just Say No' subtext, contrasts oddly with other drug stories. On the day Prince Harry escaped a police caution for smoking dope, an inquest heard how a high-achieving, 16-year-old boy on cannabis stabbed himself through the heart with scissors in front of his father.   Real life is too complex for simple analysis of danger. That is why adult stories of addiction are so sinuous. Sir Anthony Hopkins cannot resist the suggestion that his life was once enhanced, as well as ravaged, by drink. Martin Broughton, who heads British American Tobacco, concedes that smoking is bad for you and he would counsel his children never to take it up.   Such mixed messages typify our ambivalence. Print the anti-smoking notices small on cigarette packets. Downplay the charge that alcohol abuse costs the NHS 3 billion a year. But stress the parable of Rachel Whitear, an emblem of the tragedies that befall our children. Except that she, for all the prominence of her dying, was the minority of a minority; a privileged university student who found her nemesis in lethal drugs.   Hers, in the end, is a personal, not a public lesson. Her parents longed no doubt that other children should not die needlessly. They must have yearned to give a purpose to an abbreviated life and its crushed promise.   Rachel wanted her organs to help others, but heroin abuse forbade that. Instead her parents chose to breach the privacy that death normally accords. No doubt they wished, understandably, that their child should not live and die in vain.   It is indisputable that newspapers should publish pictures offered to them. It is also right that editors should understand that such stark images may have less to say about public duty than about private despair.