People who try to debunk the theory that Paul is dead & was replaced often use "fades," which are not so much fades as morphs. Morphing technology can turn one image into any other. Faulisdead on Youtube posted this video, which illustrates the point:
This fade makes it clear that the features do not match at all. The picture of Faul above also illustrates how images of Paul & Faul have been blended. For example, here is a picture of Paul on the left, Faul in the middle, & a blend of the 2 on the right (also from Sgt. Pepper):
The best method to compare images is side by side, in my opinion, since it gives the eye a chance to register the difference in facial features. If one had not been conditioned into thinking both of these men were "Paul McCartney," it would probably be shocking to think that they are supposed to be the same person.