Background image resizes along with browser

Hi,

I have been trying to get http://ilivebytheriverblog.wordpress.com to look the way I want but one thing is bugging me: when I resize the browser window or zoom in and out, the background image changes with respect to the main body of the blog. It also looks different in other browsers (I use Firefox) and doesn’t look right on tablets etc.

At the moment I haven’t paid to have access to the CSS sheets, although I have a very basic idea of CSS and have used it to do a few other minor things to the site etc.

Is there something I can do?