1. An image determining method, applied to an image sensing apparatus comprising a lens and an image sensor comprising at least one first pixel and at least one second pixel, wherein a first part for each the first pixel is covered and a second part for each the second pixel is covered, the image determining method comprising: (a) applying the first pixel to generate a first object image of an object; (b) applying the second pixel to generate a second object image of the object, wherein the first object image and the second object image form a first combined object image; (c) computing a first brightness information variation tendency for the first object image, a second brightness information variation tendency for the second object image, and a brightness information variation tendency for the first combined object image; and (d) determining if the first combined object image is a front image or a rear image according to the first brightness information variation tendency, the second brightness information variation tendency and the brightness information variation tendency for the first combined object image.