You can probably find this information of the rocket theme forum but I thought I'd put how I resolved this here.

 The issue is basically that you want the background to stay in its position in the browser while the rest of the elements scroll about.

  1.  Go to Extensions -> Template Manager> Edit the CSS for the template
  2. You should be given an option of css files, select styles.css and click on the Edit button 
  3. Look for these lines: 
             body.bgstyle1 #main-background {background: url(../images/backgrounds/bgstyle1/bgimage.jpg) 50% 0 no-repeat;}
  4. where 1 is the number of the style of the template I am setting the background for and I add the "fixed" attribute to this id.  This will yield the following: 
    body.bgstyle1 #main-background {background: url(../images/backgrounds/bgstyle1/bgimage.jpg) 50% 0 no-repeat fixed;}��
  5.  Click on the 'Save' button.
  6. The theme is getting it from a generated php file.  Look in the CSS folder of the template.
  7. You should find a PHP file in this folder with the body.bgstyle# line you need to change as in steps 3 and 4.