My desktop BG isn't going on the entire screen

[XperiaAir]

It is like this
http://gyazo.com/a6530a4c72277c48079c208d088ffca8

 

[wanderer11]

The picture is either not the same resolution as your monitor or you chose the wrong option when you set it as background.

 

[Guest]

It is like this
http://gyazo.com/a6530a4c72277c48079c208d088ffca8

Depending on your version of Windows, right click on the desktop go to wallpaper, or personalize and either use the fill or stretch option. If it is a lower quality pic it may become very blurry. Its best to search for an image with the same resolution as your desktop for optimal quality.