This tutorial shows you how to use your own picture or any picture as your start up background in Windows 7, 8, or 8.1. Its very simple and straight forward.
Learn how to customize your Windows 8.1 Start screen and Desktop screen to fit your needs. Change background pictures, windows colors, fonts, and more.
From HP Computing Support
Here are the Steps Which will guide u through The Process. The Name of the files are user.png, user.bmp, user-200.png and user-40.png for Windows 8 & 8.1.
Windows 8 is not preferred by some users because of its lack of customization options. How to bypass this limitation? You'll have to use third-party apps like ...
From Windows and Linux Tutorials from Howtech