I first saw this one in the Facebook app. But, I’m not sure if they’re the first ones to introduce it. I do know that once I got used to the gesture I was very annoyed at any app that didn’t implement it.
After tapping on a photo to bring it up full-screen, instead of tapping a “Done” button somewhere, you simply swipe up or down as though you’re flicking the photo out of the way. Since I first remember encountering it, swipe to dismiss photos has been implemented in Twitter, the iOS photos app, Pinterest, and many others.