This is not the very best MM episode; "The Killings at Badger's Drift" and "The Green Man', for examples, are much better. But 'Picture of Innocence' is a near perfect exemplar of the theme of Midsomer Murders. Because there is a consistent thread in this series of old vs. New, city vs. Country, quaint vs. Techno. If you don't see this, you've really missed the point. (This is not, of course, the only episode that encapsulates this theme. But few do it better.)
We don't just have a conflict between analog and digital, digital vs. Film. We have the modern conflict between rural vs. Urban in miniature. The maguffin of photography obscures the the actual tension between tradition and modernity. Modernity is personified by a street gang of young digital photography buffs, an hysterically funny concept. Law and order is represented by the middle-aged (and older) traditionalist film photographers. It's the kids versus the squares!
The plot is just a vehicle to lampoon the clash of these two cultures. The leader of the digital pack is wonderfully limned by Andrew Tiernan, an actor of tremendous ability, while the "squares' are represented by Adrian Scarborough, an actor for all seasons.
The story itself is nifty, but really just framework on which to hang the showdown of old vs. Young. The modern vs. The traditional. In the event, it is a traditional film shot that points to the solution. But that's all window dressing.
While there is precisely one person worthy of our sympathy, sympathy is not the point. Which is that neither generation comes off covered in glory. Life is messy, progress means losses amidst the gains, nothing worth having is free. Trite, perhaps, but it's the foundation of a quite enjoyable episode.