Free desktop wallpaper can be hard to find and even harder to find at the same size as your computer or laptop screen. When you search for free wallpaper in a standard search engine you will find very few quality images and when you do they will most likely be the wrong image size and it will look pixelated when you set your background as the desktop wallpaper image you want.
Right-click your desktop. Click Properties. Click Settings. Look at the screen resolution bar and see what it is currently set at. Adjust the bar to the level you like (preferably more as the higher the resolution the better quality the wallpaper image.)
Type the resolution number your screen is at into a search engine like Google. For example type "1680x1050 wallpaper" into the search engine. This will find you specific desktop background wallpaper images you can save and then set as your background.
Search the websites listed under the additional resources below to find free wallpaper for your computer screen background that are free and are sorted by screen size. Save images you like and right-click them and set them as your background picture.