Well, besides they're heads being a bit big, and Vegeta looking on the skinny side (which I like ;) ) it's a very good picture. You really captured Vegeta's personality, and Goku's expression is priceless. I don't know when I saw a DBZ pic this good.
This is an awesome pic. I didn't actually notice the head porportion until I read your comment. What I did notice was Goku's right arm- the one closest to us in the image, looked a bit off. Besides that I love it!