How to change Account Picture, Lock Screen and Start Screen Wallpaper in Windows 8?

To Change the Account Picture:  

1. Press the Windows key to launch the Start Screen.
2. Click on the User Tile on top right corner of the Start screen.
3. Select Change Account Picture.
4. Click one of the provided background images, or use the Browse button and select any image from your computer, Bing, SkyDrive, or even your camera.

To Change the Lock Screen Picture:

Lock screen settings are located in the PC setting application.

1. To access it, open the Settings charm. You can press Windows Key + I to quickly open the Settings charm from anywhere in Windows.
2. Select Change PC Settings.
3. Select the Personalize category and select Lock screen.
4. Click one of the provided background images, or use the Browse button and select any image from your computer, Bing, SkyDrive, or even your camera.

To Change the Start Screen Wallpaper:

1. To access it, open the Settings charm by pressing the Windows Key + I.
2. Select Change PC Settings.
3. Click on the Personalize category, then click Start Screen and select the background image and color scheme.