With PR, it should always be about finding people who are genuinely interested / excited about what you're doing, so I would suggest targeting those people regardless of whether they are nice, hyper-local, or broad. Leave no stone unturned but it really depends on what you can use as a hook, which without understanding where you're exactly at, can't help beyond this general advice.
One thing I'd add as a note of caution is that if you're not already trending towards 100,000 installs since US app store availability, it's going to be very difficult to get press coverage absent a very unique hook.
Happy to talk to you to try and help identify what hooks you might have.
Best of luck!
I'd be surprised if press releases for yet another a photo sharing app can do more than create a state of background brand awareness, and that recedes rather quickly.
Actually receiving and sending photo messages is how you'll be discovered and remembered -- that is to say, socially and virally.
I remember first hearing of Facebook when it was young from a long-distance friend who wanted to share a photo with me. Like most Facebook users, I never read any press releases about the fledgling company; and if we had, it probably would have had negligible effect. A press release may be little more than a check in the box -- something for corporate investors and bloggers to refer to later.
If it were me, I'd put little weight on the PR and concentrate instead on sharing your product with well connected folks with non-overlapping audiences -- bloggers and tweeters who naturally like experiment, create some buzz, and share photos.
Oddly enough, I've daydreamed some very odd techniques for running a promotion through viral photo sharing. I'd be quite interested to put my experiment to the test. So if you ever get to that stage and don't mind hearing about something well outside the box, find me and ask.
Press releases are a good start...but not really a full launch strategy. It takes a bit more coordination and intent. This link (http://untether.tv/2014/pr-mobile-application-cheap/) is to a video that highlights a few concepts on how to launch right with PR and this link (http://untether.tv/category/mobile-app-minute/) has 20 other similar episdes on how to launch and market your mobile app.
Hopefully these help! Good luck.