Hey Pete,
It's a great question, and I'd love it if someone with a case study would chime in here, but I've never seen one done on this topic (would make a great blog post if you had proof one way or the other, huh?).
In my experience, I've not seen shared photos between listings have any noticeable negative impact on listings, nor can I see that having unique images has had a positive impact, but as I've said, I've never done a side-by-side study of this or seen one published.
The only reason I can imagine for fearing a negative impact of this would be increasing a risk of merging between multi-location business listings, but I strongly doubt Google uses image file names as a differential factor. So, I really wouldn't be concerned about this.
I'll also add ... I've been a participant in Local Search Ranking Factors since year 1 and I've never seen this cited as a strongly-felt negative or positive ranking factor.
So, bottom line, my gut feeling is that if this comes into play in any way, it is so minor that no one has documented it.
Looking at this from a non-ranking perspective, however, we can make the argument that unique photos might positively affect conversions. Let's say your client is a realtor. If he puts the same 5 generic images of houses across every single Google+ Local listing his company has, the impact might be less local feeling for human visitors than if, say, his San Francisco branch shows some of those famous city Victorians while his Palm Springs page shows some classic Mission-style houses. Some industries may have stronger visual queues than others - so there is likely some grey area here. What I would say, in conclusion, is that I would consider Google+ Local photos from a usability/conversion perspective more carefully than I would from an SEO perspective.
I know I've seen discussions on fora of how images on Google+ Local pages may influence conversions, but I'm not finding any easy-to-hand. This might be worth your researching further. I hope you'll get more feedback from our community, too:)