"This story is very damaging for her," veteran London-based public relations expert Max Clifford told CNN.
"There has been nothing but a honeymoon for Pippa since last year's royal wedding so this is dreadful," Clifford said. "It comes particularly at a time when guns in Paris are such a sensitive issue. The reality is she could have been shot by police."
Middleton has been a target for tabloid newspapers and paparazzi since her sister married Prince William last April. Shortly after the wedding, personal pictures of the 28-year-old began to leak out to the press, including a photo showing her in a bikini while on vacation in 2006 with her sister and Prince William. A photo of Middleton sunbathing topless on that trip was also made public. Later, undated photos from a private party showing her dancing in her bra were also leaked.
The latest incident happened as paparazzi were taking pictures of Middleton who was being driven to a Paris train station in an Audi convertible. The car's driver wielded a gun, apparently pointing it in the air and then at a photographer, who was following them and captured the shot, according to The Sun. It is unclear if the gun was real or fake.
Lawyers told CNN that Middleton was unlikely to face charges unless it could be proved she encouraged the driver to point the gun on public streets, but the driver could face charges of threatening with a firearm if French authorities wanted to do so.
Middleton is the sister of Catherine, Duchess of Cambridge, who married Prince William last year. William is second in line to the British throne behind his father, Prince Charles.
Buckingham Palace said it would not comment on the photo because Middleton is not a member of the royal family.
The photo is causing a stir, after a spate of gun violence in France recently, including a series of attacks last month that killed seven people, including three children.
The incident took place while security was heightened ahead of France's presidential election.
"Pippa must understand she's in the middle of a media spotlight and she can't just turn the attention on and off," Clifford added. "I assume she has PR advice but either she's getting incredibly bad advice or she's not listening."
The story has been splashed across newspapers around the world, and on Tuesday royal experts warned that Middleton must avoid further incidents for risk of damaging the royal family.
"Pippa is surprised at how high-profile she now is," one royal newspaper correspondent, who has met Middleton on several occasions, told CNN. "Part of her enjoys the attention: she's always mixed with high-society characters and she loves partying but she's not off the rails by any means.
"This prank in a Paris street will come as a real wake-up call for Pippa," the correspondent, who declined to be named, told CNN. "She must now realize that it just takes one person waving a gun around to seriously dent her public image. She'll be gutted by how badly this has reflected on the royals."
But the correspondent said he believed Paris authorities would have no choice but to investigate the incident, while Middleton would be forced to issue a public apology.
The photographer who snapped the shot was "considering his options" about a formal police complaint, The Sun reported.
The Daily Express's royal correspondent Richard Palmer commented that ever since Middleton became last year's most celebrated bridesmaid at the wedding of her sister Kate to Prince William, she has been "riding the crest of a wave of popularity. Some think she has also been riding for a fall."
Palmer said Middleton was popular at high-society events, but she and her friends needed "to learn how to behave in front of the cameras in return."
The incident "will have embarrassed her sister Kate, her brother-in-law Prince William, and also the Queen in her Diamond Jubilee year."
"She need to explain herself and apologize."