DOC: Updated the highlights to promote the release and the upcoming paper

Hello all,

I am getting ready to send an e-mail about the dipy 0.7.1 release across all the lists and people who have shown interest in Dipy and I thought of changing the highlights sections in the website with promoting the release and the upcoming dipy paper as we expect many visitors. 

The problem I have at the moment is that I want to have this picture (named dipy_paper_logo.jpg) to be bigger and centered when there is space (e.g. the browser window is maximized) and smaller when the page takes less space without obstructing other areas of the website (e.g. the table index). I have here a naive solution using html but probably this is a CSS issue which I am not familiar with. 

I would appreciate very much if someone could help me with this.

GGT!