Clickable image as background

Hi! Love the theme so far, been easy to use and easy to get it to look great. My site: obsid.se I've come to a problem I can't seem to fix though. 1: How large would I have to make a background image for it to show an ad on all sides of the content when looking at the site from a computer? I want to use it as ad space, but Im not sure how large to make the image. 2: How can I make the image into a link to my chosen destination? My site: obsid.se Thanks in advance!