An iPad app
Run the standard app on an iPad and you can actually see the photos. And all it’s doing is pixel-doubling (as far as I can tell). If your medium is imagery, go to the platforms that make them the best as they can be. Also: The iPhone 6 Plus seems like it would require them to be thinking along the larger screen line, anyway.

Pinch to zoom
I want to see more of some photos. Let me pinch it to zoom in and out.

URLs in comments
I mean, come on.

Comment reply notifications and likes
Facebook figured this out a long time ago. Let me know when someone else comments on a photo I’ve commented on (and let me opt out of those notifications if I want). Let someone like my comment if that’s all they want to do.

BONUS THING: A new app icon
I mean, come on.