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Paul is Dead: New evidence based on computer enhanced forensic techniques

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The two researchers began their analysis by obtaining high quality photos of Paul McCartney before and after the car accident. Gavazzeni complained that some photos taken before 1966 were not properly dated (sometimes different photo agencies would have different dates for the same photo), and that some of the best photos were the property of photographers who were very reluctant to release them. However, they eventually found two good quality photos dated before 1966 and two after 1967.

Different photos of a human face, in order to be compared, have to be re-sized to match the same scale; to do this, one feature of the face must be used as a scaling factor. 
These two researchers opted for the distance between the pupils. In other words, the photos were scaled so that the distance between the pupils was exactly the same for all of them.

Once photos of a person have been scaled so that the eyes have identical locations, then if you lay the photos on top of each other, certain features will match exactly, such as the shape of the skull. The skin and hair may be very different, but the skulls should be identical.

However, the researchers were shocked to discover that the photos prior to 1966 did not match the photos taken after 1967.

For example, the frontal curvature of the jaw was different (i.e. the curve going from one ear to the other and passing through the chin, which you see when looking directly into a face, as in the photos on the right) and the jaw arc was also different (i.e. the curve of the jaw that you would see if looking downward at the head from above).

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(Source: erichufschmid.net)