How can I set a custom image as the Windows 8 Start Menu background?
You can do this using the Windows 8 Start Tweaker, scroll down under the images to find the description and information and at the right side of the page you'll find the download button / link.
Download Windows 8 Start Tweaker zip file. Right-click on the zip file and select Extract all option to get Win8StartTweaker.exe file.
On the first run, you might see the Chinese language. Worry not! Click on the US flag (located at the bottom right) to change the language to English.
Click on the default Start screen background image to select a new image file. For best results, select an image file whose height is double than the screen resolution. For example, if your screen resolution is 1440 x 900, then the height of the image file should be x1800 px.
Click on Metro background image to set a new Metro background color. This color covers the logon screen, Start screen search and some other areas.
Finally click on Apply button to see the change. Click Yes for the prompt to automatically log off and log on your system to apply the changes. Even though the tool lets you restore the original background and color, do make a system restore point so that you can revert to the original Start screen background and Metro color without any issues.
— Source: Into Windows - How To Change Windows 8 Start Screen Background