I would say 'An' is fine in this use, though 'The' would also be appropriate. The 'exclusive' part of the quote suggests that there was only a one off occasion where photographs were taken.
'The' would be used to add emphasis, stressing that the photoshoot is widely known and important, an event that everyone knows about.
Like I said not a teacher only a native, perhaps a teacher could add some insight.
Student or Learner