Trace back the digitally altered photograph of Catherine, Princess of Wales, and its roots lie in a tragedy of another Princess of Wales, Diana, whose death in 1997 predated the creation of Facebook by nearly seven years.

Diana’s fatal car accident, after a high-speed pursuit by photographers in Paris, left a lasting imprint on her sons, William and Harry. They grew up vowing not to take part in what they viewed as a pathological relationship between the royal family and the press, one in which they were the abused partners.

The rise of social media gave this younger generation of royals a way to bypass the tabloids they reviled, with popular platforms like Instagram and Twitter, where they could post carefully curated news and images of themselves, unmediated by the London papers or the lurking paparazzi.

But now they are experiencing the darker side of public life in the wild west of the web. Catherine’s photo, posted on social media and picked up by newspapers and broadcasters worldwide, has been swept into the maelstrom of rumors and conspiracy theories that have haunted her since she underwent abdominal surgery and receded from the public eye two months ago.

While William and Harry have struggled with these forces, the pressure has perhaps been most acute on their wives, Catherine and Meghan, who have taken turns being in the eye of an online storm. Meghan spoke recently about the “hateful” treatment she experienced while pregnant with her children.

“It has to be so hard to deal with this,” said Rasmus Kleis Nielsen, the director of the Reuters Institute for the Study of Journalism at Oxford University. “It is often women who are subjected to the worst bullying and harassment.”

Catherine, of course, inadvertently contributed to the hothouse atmosphere by altering the Mother’s Day photo of herself and her three children. That unleashed a fresh storm of online speculation, with people sharing pet theories about how the image had been manipulated, whether by transposing Catherine’s head from a 2016 cover photo in Vogue magazine or recycling a shot of the family taken last November.

Visual investigators debunked both suggestions, but that didn’t stop the original posts from going viral, with one post promoting the Vogue theory racking up more than 45 million views.

Having set out to control her image, Catherine now finds herself in a predicament not unlike some of her royal forebears, dogged by an online pack no less feral than the photographers who chased Diana in Paris.

“Anyone in the royal family or their staff who think that social media allows people to circumvent gatekeepers or control the narrative has not been paying attention to Meghan Markle’s experience,” Professor Nielsen said.

“These are deeply ambiguous spaces,” he said, “in which things that people want are inextricably linked with things that are deeply troubling.”

William and Harry made their first official foray into social media in 2015, when they, along with Catherine, opened shared Twitter and Instagram accounts. An early post showed Harry, standing on his tiptoes, next to the 7-foot, 2-inch retired American basketball star, Dikembe Mutombo, at a coaching program for young people.

When Harry met Meghan, an American actress, the following year, he was exposed to an avid and expert user of social media. Meghan was running a lifestyle blog, The Tig, which she described as “a hub for the discerning palate.” Cosmopolitan magazine once said it was “well on the way to becoming the next Goop,” the wellness brand owned by the actress Gwyneth Paltrow.

Meghan shut down The Tig after her romance with Harry became public. But she brought her new use of social media with her into the royal family. When the couple announced in 2020 that they planned to withdraw from royal duties, they broke the news on Instagram and laid out their plans on Sussex Royal, a site designed by the same Toronto-based digital firm that had designed The Tig.

When Meghan was targeted by abusive language online, Harry blamed it on hostile and racist press coverage. In his memoir, “Spare,” he wrote that, in the 18 months before his 2018 marriage, relentless tabloid coverage of Meghan had “riled up all the trolls, who were now crawling out of their cellars and lairs.”

“Ever since we’d admitted we were a couple,” Harry said, “we’d been flooded with racist taunts and death threats on social media.”

Yet, in Catherine’s case, the lack of press reports may have contributed to the rumors mushrooming online. Kensington Palace, where she and her husband, William, had their offices, erected a veil of privacy around Catherine after her surgery, offering few details about her condition or recovery, beyond saying she would return to work after Easter.

“The near silence over Kate’s health, which she was perfectly entitled to observe, spooked the media and generated a social media frenzy that the mainstream media fed off,” said Peter Hunt, a former royal correspondent for the BBC.

For all the ravenous coverage of the royal family, some subjects are off limits. Gossip about William and Catherine’s marriage has long percolated in the web’s murky depths, for example. But it rarely, if ever, surfaces in the papers, which adhere to strict guidelines on privacy, enforced by Britain’s robust defamation laws.

When a grainy photo of Catherine riding in a car with her mother appeared on the American gossip site, TMZ, last week, British papers did not publish it out of reference to Kensington Palace’s appeal that she was allowed to recover in peace.

Even now, in the wake of Catherine’s confession that she had retouched the photo, a few tabloids have rallied to her defense. “Lay off Kate,” read the front page of The Sun, which is published by Rupert Murdoch and typically gives the princess generous coverage. “Attacks over edited pic are absurd,” it added.

The risk for the royal family, experts say, is that Catherine’s manipulation of the photo will cast doubt over other news and images released by them, depriving the royals of a useful channel to reach younger people. Some tabloids were openly skeptical of her. “How did Kate Photo Become a PR Disaster?” asked The Daily Mail. “Kate’s Photo Bomb!” declared the tabloid, Metro.

“Social media should be a win-win for the royals, a means of disseminating their unchallenged and unjustified message,” Mr. Hunt said. “While most will probably forgive and forget, the risk is an erosion of trust, an important commodity for the monarchy.”

The royal family’s credibility may not be the only casualty. Professor Nielsen noted that, in a recent survey, 69 percent of people in Britain said they were concerned about what was real and what was fake on the internet. And that was before the whirling vortex of rumors and misinformation about Catherine.

“This may well further intensify people’s skepticism toward much of what they see, both from news media and on social media,” he said. “Not a great few days for people’s confidence in the information environment.”