On the right side of John Lennon's face, you need to bring the jaw line up a bit and narrow the face - see how that side of his face looks much wider? It's the jaw line that does that, it needs to be at more of an angle, like it is on the other side, and that will help a lot. And it's hard to tell from the angle the picture is taken at, but I think more hair - go further up on Paul.

One good thing to remember drawing people is that, when the oval of the face is drawn, the eyes - the middle of the eyes - go on a line drawn in the MIDDLE of the oval. That's not exactly true for every face, but it works for almost everyone. So measure it out that way, and add more hair on the top for Paul to make the proportions better. There are other guidelines - like the pupils of the eyes generally align to the corners of the mouth, etc., I could bore you for hours, but they are helpful basics.

Have you tried Google's Image Search for good Ringo Starr pictures? It's usually good for that!