Yes. The Windows 8 Start screen background can be changed. Here is how to do it.
Windows 8 glaring feature is its Start screen with a background and tiles over it. There are options to customize the colors and background with Microsoft defined colors or images only.
Here is how to have your own custom background in Start Screen.
ModernUI Start Screen Changer designed by vhanla @deviantart is the utility which helps in customization.
It is a freeware utility and a portable app (no installation needed) for Windows 8. The background can be selected by just adding the custom image.
The following screenshot clearly explains how to do : (make sure that the image is a PNG picture)
How this works
The tool modifies the imageres.dll file which maps what picture is the current background for the Start Screen.
The way an image is fetched is by splitting it into two pictures with a rift at the middle behind the tiles. This split is not noticeable with the Windows given default images. However with our own images it might be visible. So, a full background image is not possible as Windows 8 design is that way.
For example, in a 2000×400 resolution image, it is cut into two with the top part being 2000x264 size and the bottom part 2000x136 size.
However, this utility seems to be the only freeware currently available for background customization.