On the face of things, it looks like a happy family snap.
King Juan Carlos of Spain sits with the small Prince William, while a radiant Princess Diana, a protective arm round toddler Prince Harry, leans in to share a pleasantry with the good-looking monarch.
But look again. At the other end of the couch, Prince Charles seems scarcely part of the same holiday party in 1986. He is staring glumly straight ahead like the proverbial gooseberry.
Princess Diana is rumoured to be just one of the many young ladies the king, now 74, pursued in a romantic career in which - like his namesake, the seducer Don Juan - he is said to have bedded more than 1,500 women.
The explosive claim is made in a new book by Barcelona-based author Pilar Eyre, who has already written six volumes about the Spanish royal family.
Imperious and suave, Juan Carlos looks every inch the old-style monarch with the autocratic manners to go with it. He loves hunting bears, skiing and boating - and bedding the opposite sex.
|Distant: Prince Charles stares into the distance in this 1986 photograph showing him, Diana and the young Princes William and Harry with Queen Sofia and King Carlos of Spain
Apparently, it is an open secret in his circles that he is such a keen womaniser that the only woman he does not spend much time with is his wife, Greek-born Queen Sofia. According to Eyre, the two have not shared a bed for 35 years.
In fact, the book says, following an operation on a benign lung tumour at a Barcelona hospital in 2010, the woman who spent most of the time consoling him during his convalescence was a 25-year-old German interpreter called Corinne.
But can it really be true that our very own Princess Diana was one of Juan Carlos's most significant conquests? And that it was the relationship between her and the then 48-year-old king, in the prime of his romantic life, that finally put paid to any chance of reviving his marriage?
It is certainly the case that the Princess, together with Prince Charles and their young children, holidayed in Majorca with the Spanish royal family several times during the Eighties.
Charles never felt at ease on the sunshine island and much preferred visiting the Duke of Wellington's estate near Granada on the mainland where the shooting was good.
But Diana, who loved lounging about on yachts in stylish bathing suits, was right at home on the shores of the Mediterranean where she could show off her figure. And the king, who appreciated displays of female beauty, seems to have acted on an impulse to get closer to her.
After her first trip to Majorca in 1986, Eyre alleges Diana told her bodyguard Ken Wharfe that Juan Carlos fancied her. Apparently, the king made all sorts of excuses to get tactile with her and used to love bending down with her and inviting her to stroke his old German shepherd dog, Archie.
Another royal biographer, Lady Colin Campbell, has long insisted that the Princess and the king embarked on an affair while on a cruise with their spouses in August 1986, and that they took up with each other again the following summer.
'Diana did it to make Charles jealous, but it didn't work,' says Lady Colin. 'Charles couldn't have cared less.'
According to Eyre, rumours of the affair intensified later over the curious case of some photos of Diana in a state of undress. These were touted around the world's publications, only to be taken off the market when someone in Spain paid $45,000 (£29,000) for them. That someone is rumoured to have been Juan Carlos, who wanted to protect the Princess's reputation.
But why rake all this up now? Diana is long since dead, while Juan Carlos, though he retains an eye for a pretty woman, has made it quite plain that he would never divorce his wife, with whom he has three children and eight grandchildren.
Eyre says she has revealed it for Queen Sofia's sake. 'In a macho country like Spain, the king's womanising image makes him very popular,' she says. 'Even the women don't reproach him. On the contrary, they love him because he has such a seductive manner with them. But they don't feel the same about poor Queen Sofia.
© Daily Mail, London