Remove background from several weather images

A few of the cloud images weren't fully transparent.

![Before](https://f.cloud.github.com/assets/240516/377974/fdbaeaf4-a506-11e2-8410-12920b11a8e0.png)
Before

![After](https://f.cloud.github.com/assets/240516/377975/fdc99d38-a506-11e2-9de7-917ef3a51a7e.png)
After