Preserve background transparency during image composition

Demo Image:
![demo](https://f.cloud.github.com/assets/202773/1861994/7be17678-77ca-11e3-826d-030606b48aa2.png)

Original Image: 
https://f.cloud.github.com/assets/202773/1861998/8b6718a0-77ca-11e3-90d6-194007c3615b.png