You can try adding a bit more variation on the oppsite side of your tree, as Scott sugested. You may also wish to use colors that have a bit more contrast, as it stands they are a tad bleak. However, before you do any thing, I would sugest you invest some time into learning more about pixelation, it appears to be the style you're using, and the more you know about it, the better the results you will get from your efforts. You can find a lot of good tutorials about pixelation at
http://www.zoggles.co.uk/asp/tutorials.asp?show=index