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.
This video tutorial provides those new to Windows 8 a basic introduction to the new Windows 8 user account management utilities, like the Windows 8 App, and ...
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
This tutorial shows you how to set the Windows 8 user account picture back to the default one.
From Simple Tech Tutorials