You can't put your own photo in a Windows 8
Start screen background because the built-in customization limits the pictures to the Microsoft-provided ones. However, StarDock has released a program called Decor8 that allows any picture to be used and is available from the StarDock website
. This allows your Start Screen to change from this:
Windows 8 Start Screen before StarDock
Windows 8 Start Screen after StarDock
Note that you need to click the Select folder button at the bottom of the Decor8 configuration screen to add additional folders to use as images as shown.
Decor8 Configuration Screen
You might also be interested in the answer to this FAQ "Q: How can I add Shutdown and Restart tiles directly to the Windows 8 Start screen?"