bwaaahhh!
Here's the problem, or at least, here's how I see the problem. Alyson is a skinny girl, with a face to match. You're trying to put her skinny girl head, on the body of a heavy girl. The result is bound to look out of place. Michelle, had a bit more roundness to her features, so a fuller bodied swap doesn't looks so out of place.
And now for a general word on a fulfilling career in face swaping:
1. Pick the right head for the right body. Try to find a donor body, of a girl with a similar head shape, size length... This was just touched on, but it really makes everything MUCH easier. For example, Natalie Portman looks great with Christinamodel, their heads just mesh really well. (making Natalie that wierd orange color is a bitch though)
2. Placement. This can be tricky. Try and find a picture where the "new face" is in the same position as the old one. As a rule of thumb, I like to use the ears as the "lining up" point. The first reaction is to use the eyes or the jaw, but this will result in an off looking picture, because not everyone's eyes are equally distant in relation to their head, same of jaw. But, eveyone's ears are more or less where their head meets thier neck. If there is excess jaw hanging out, you'll need to clone over it. Of course, this is rarely perfect. Which segues nicely into the next part...
3. THE NECK! When a head is in a different position, the neck will fold differently. Study some anatomy and try to recreate any new folds, trachea bulges...
4. Hair. This can be another pain; but, it can also become a friend to cover up flaws. If someone's head is looking to flat, not enough crown, neck problems, add some more hair! Find donor hair from another picture if needed. Of course blending hair can be arduous, but poorly blended hair can be a crappy picture. (Be sure to test the image at different brightnesses, Many A time I make a pic on one monitor and it looks fantastic, only to see it on a different monitor, and I notice a bunch of hue, brightness, and sharpess differences that make the image look like crap) One last, but important thing about hair. Always keep the hairline of the person who is being portrayed as the fake. A picture of Aly Hannigan, should have Aly Hannigan's hair line. Otherwise it looks like an amalgm rather than a fake. Bangs, or any hair peeking over the face is the only exception to this.