Here's how you can use the same background image on your desktop and Start screen in Windows 8.1.
1. Go to the Start screen.
2. Put your cursor into the lower right corner.
3. Click on Settings.
4. Click on Personalize.
5. Click the desktop background image to change it to the same one you use on your Start screen.
Some people like having different backgrounds for desktop and Start, and that’s cool, but personally, I like using the same background on both. It makes for a smoother transition when I switch between the two.More at Windows