Reminder also that whilst it's a very fun story, this claim was:
a. written as part of a hit job by two right-wing Brexiter journalists in the run-up to the referendum as part of a wider effort by the right to discredit the centrist establishment in the months before the vote (see also: Work and Pensions secretary Iain Duncan Smith resigning from Cameron's cabinet in protest at the work and pensions policies he had conceived and implemented over the previous six years...); and
b. never substantiated by any evidence that those two Brexiter journalists were willing or able to provide, despite them claiming in the book there was a photo.
There are totally things Cameron should be criticised for over his time in government, but that is no excuse to parrot Brexiter (and possibly Russian) fake news that was designed to discredit moderates and favour the far-right.