the portrait of kiera knightley has a really good likeness.
i think the bottom two are better than the top one. i think with the top one because you painted on a white background with brushes on fairly low opacity, the background shows through too much and makes the skin seem really pale.
Definitely add some tone to the background. These Aren't that bad at all, especially the last two because of the better atmospheric BG. Just study the variation of skin tone more carefully and also try to find areas where you can add more contrast in your pictures, most of them, especially the first two(since they are so light on a white background) seem pretty flat and boring.
The chair behind the first one seems to almost be just one color and doesn't really help in projecting the picture much either...