Renaming banner image

I just noticed that Adblocker/Privoxy/etc. are blocking the top background image, because it has word "banner" in it.

![adblocker](https://cloud.githubusercontent.com/assets/812976/4317848/c7b65b9e-3f17-11e4-898f-c65e7aa4cfec.png)

This PR is a quick fix for it.