Using Photography for marketing can certainly be very effective.
Content and their ads on social networks can generate some market however engagement for continuous usage of services is critical to success of client's venture. Ads can't buy engagements unless product services & features are creative or outside app marketing campaigns (instead of ads) are gamified in a indirectly relevant mechanism.
The most activity mobile app user does after texting and phone calls is Take Photos.
I would suggest to create a gamified campaign around taking photos of landmarks and reward those prospective users for taking 3-5 photos, social sharing etc.
Reward something simpler like what they can physically take home or share or use while traveling etc.
now once the campaign is over, notify all the participants to social share and also introduce your actual client's product/services. There will be a great chance of these users becoming your client's genuine customers and continue using your product.

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Good luck.
Best,
Shail