Until now, pressing the back navigation button has been the only way to get out of an image in the Google Play Store on Android, but it seems a new method is rolling out. A gesture to swipe down on an image to exit has been implemented, which is certainly more intuitive.
The Play Store app is in a continual state of flux, with various UI changes rolling out to different users at different times before potentially disappearing again, so we can't say with any confidence this new gesture is here to stay. Take a look at it in the video below, provided to us by Ivan.
It's not reached any of my devices yet so it's likely to be a server-side change that may or may not be going out to only a test group for now. It might be there for you, though, if you're lucky.