Sitebuilder Plus: Resizeable Background Image

You can achieve a background image that scales to the browser window via code.

Try putting the following code in the HTML Head tag section while having an image selected to be the background. Note: This will stretch the image to be the background, so if the image is small, it may appear pixelated, and will need to be placed on every page. I would recommend doing this with a large photo. 

<style>body {      background-repeat: no-repeat;
      background-size: cover;
      }
</style>