According to the Daily Mail, Unlawful Killing will feature a black and white paparazzi image of Diana taken moments after the Mercedes carrying her and Dodi Fayed crashed in Paris in 1997. The image has not been previously published in the UK.
A spokesman for the filmmakers said: "The picture has been published in full before, in many parts of the world. We acquired the image from an Italian magazine, which had already published it in full.
"It is also widely available on the web. We are therefore not publishing anything that the rest of the world has not already seen elsewhere."
The film will premiere in Cannes on May 13.
Allen has reportedly been working on the film for three years and recently told The Guardian: "I asked every major UK broadcaster (BBC, ITV, C4, Five, Sky) to commission a TV documentary about the inquest. But they refused even to contemplate such a suggestion.
"[Dodi's father Mohammad Al] Fayed offered to fund our project, so we could make a feature-length cinema documentary instead. We agreed, on condition that we would report events in the way we saw them, and the deal was struck.
"British lawyers insisted on 87 cuts before any UK release could be contemplated. So rather than butcher the film, or risk legal action, we're showing it in France, then the US, and everywhere except the UK."
A spokesman for Al Fayed, who also features in the movie, said: "He was not aware that any photograph taken of any occupant of the car was going to be in this film. He is appalled by that and will be taking all necessary steps to make sure it is not in the film."
Watch the Unlawful Killing trailer video below