I can mask an image in Sketch by selecting the image and then clicking the "mask" button. I then have the ability to define the shape of the masking object. After the image is masked, if I double click the image I have the ability to edit the image inside the mask: resizing it, etc.
But how do I edit the shape masking the image, after the mask has already been applied? If I try to click the invisible shape masking the image, and resize it, nothing seems to happen (often the program thinks I want to select whatever object exists under the one I'm trying to edit). Every once in a while, I can successfully select the masking shape to resize it, but there doesn't seem to be any consistency with that ability and I have no idea how I trigger it.