Your Canvas is the parent, your Image should be a child object. My screenshot would be the settings of the child, which stretches to fit the size of the Canvas.
Those settings in the Game tab will let you preview what your UI will look like using different resolutions, so your Canvas will resize accordingly. That is not the resolution that your game will use in a build, necessarily. The game will still try to fit whatever aspect ratio it’s displayed on, with the Canvas resizing according to how you set it and its children up.