One of our pilot phase participants asked me about making the background image always fill the screen. You can do this by adjusting the CSS. Here's how:
From your Content Management sidebar menu, click Site Background Image and click the Settings tab, or from your Administration menu, choose Configuration -> User Interface -> Dynamic Background -> Settings.
Click the "Advanced - CSS Behavior" link.
Leave .html as the selector.
In the CSS field, enter the following:
background-position:center fixed; background-repeat:no-repeat;
Click the Save Configuration button.