A developer preview version of Windows 8 is here for a while now. Though, it is released for testing purpose, there are many utilities already available to tweak Windows 8 appearance. Here is another little tool called ‘Windows 8 Start Tweaker’ that lets you simply change the background image ofWindows 8 Metro screen and color of logon screen.
In Windows 8 (Dev Preview), there is no option for users to change the Metro screen background image and start screen (logon screen) green color.
With Windows 8 Start Tweaker, you can make any image file (png/bmp/jpg/gif) as your default Metro Screen background, and any color for your Start screen or logon screen just with a click. Just run the tool, select an image (or color for logon screen) and click 'Apply' button. You can reset the changes to default by clicking the 'Reset' button.
Note: If you want the image fills out the whole screen, you should make a image by yourself. The Height of the image should be doubled with the Height of your Screen Resolution. For example, if your Screen Resolution is 1280x800, you should make a image with Height 1600, and the width is not limited.
Interested users can download Windows 8 Start Tweaker (180KB, Portable) at source here (It works for Windows 8 WDP (build 8102) only).