It's not possible by default to select a background image of your choice for Windows 8 Start Screen. You are given a limited number of colors and backgrounds. If you want to use a custom wallpaper background then you will have to use third party applications. The most popular one is Stardock Decor8 but it isn't free. There's a free trial available but the full app is available for $4.99. There is no need to post more about it since it's not free and has been covered here on Winmatrix.
If you are looking for a free alternative to Decor8, then you need ro download Windows 8 Start Screen Customizer by vhanla.
Download the free software from here. You will need 7-zip to unpack the archive. Then you need to change the file extension from .ex_ to .exe. The file extension has been modified by its author to avoid being blocked by security software. VirusTotal test confirms that the program is safe to use. Launch the exe file and click on Load Picture to select an image from your PC. Use drag and drop to move the rectangle over the part of the image that you want as your background image. Once you are done click on apply to set the new background.
Don't forget to check the "run at startup box" so that your background is set automatically whenever you start your Windows PC.