Investigators ruled the British princess' 1997 death was the result of her chauffeur's drunken driving in Paris.
Although it is well-known paparazzi snapped photos of her moments before she died, most media outlets have declined to publish the pictures.
ABC News said British lawyers have fought to ban the release of "Unlawful Killing" in England, unless the controversial images are removed.
The 90-minute documentary, which is to debut at the festival this week, was partially financed by Mohamed al Fayed, father of Diana's boyfriend, Dodi Fayed, who also died in the crash. The big-screen exploration of the investigation into the couple's deaths includes a letter Diana penned, expressing how she was afraid her ex-husband, Prince Charles, wanted her dead, ABC News said.
The public showing of the photos will likely be heartbreaking to Diana's sons, Princes William and Harry, said Katie Nicholl, author of "The Making of a Royal Romance."
"The boys absolutely abhor anything that attempts to cash in on their mother, be it a film, be it a book that attempts to capitalize on her death," Nicholl told ABC News. "It's the one thing they can be very vocal about. It's obviously very timely with the royal wedding and with Diana's 50th birthday, but it will be incredibly painful for William and Harry."
Prince William married Kate Middleton last month at Westminster Abbey, the same church where Diana's funeral was held nearly 14 years ago.