Although it isn’t obvious at first, in the Android Instagram app you can (after editing a photo) go back to the photo roll to view an old image. Then a frame appears with drag handles on the side; you can move the frame and resize it by dragging the handles, allowing you to crop to any part of the image.
How do I do this on Android? Is this a standard feature, is there a similar open source library?
Is this along the lines of what you want?