To turn a banner image ON for your asset:
The default is banner off.
To create a full width banner image across the top of your site:
When this banner image with 960 pixel width and 122 pixel height, is used with Left menu showing it will scale down to a smaller version 710 pixel width and 90 pixel height.
If you know your site will always show the "Left menu" and you want to create a banner image specific to this content width eg: similar to the one used on this page, follow the steps above but create the banner image with 710 pixel width and 122 pixel height.
NOTE: This width 710 pixel is not suitable for full width display - it will leave a white gap on the right side.
You have the option to increase or decrease the height of your images.
See "Banner Full Width"