Windows 8 Developer Preview lacks a feature named 'customization of the Metro background' that let you change the background in Metro.
"While you can easily change the theme or background of the desktop, with Windows 7 themes supported in the new desktop client, but you find yourself stuck when it comes to making a similar move with the Metro background as well."
"In the Windows Developer Preview the background of the metro start menu is stored in « shsxs.dll » as PNG resource, and their is not official way to use a custom background," winunleaked explains.
Adding, "In Pre-BETA build shsxs.dll don't exist anymore, the metro start menu background is store in « imageres.dll » as PNG resource, the metro control panel allow you to change the background, but you can not use a custom background, only one of the eight pre-defined background," the blog said.
There are eight pre-defined backgrounds available to choose from, though setting a custom image as the background might prove more than difficult to achieve.
There is no support for usual .png images, for example, but only for 8-Bit photos. Unlike the Windows Developer Preview, in Pre-BETA (and next build, maybe) you can not change with a resource editor the background with a custom png with rgb color (like 99.9 % of images), you need to use a png with 8Bits Indexed color, if you don't use this format the new background image was not display just a solid color instead. Also you can not display really an image as background, just an image >> effect >> because all colors change on the fly to fit to the metro color of your choice.
Furthermore, there is way to actually convert an image to 8-bits and then use it as a background, simply because the system will automatically convert its color to match the metro color of one's choice. Basically, regardless the selected image, you will get a green colored one when selecting green as the default Metro color, and so forth.