I want to know: if we allow a photo editing app access to the photo gallery, can it access the photos only when actively being used, or even in the background? Can it upload my photos to its server in the background? And if so, how do I know that the app is active in the background?
There are some limits to what an app can do in the background, but once you give an app access to your photos, it has that access even while running in the background, and it could upload them to a server.
To kill an app running in the background, just use the normal procedure: double-tap the Home button and swipe the application window up.