how do i change the background on my mini acer notebook?

kay so I have my laptop to Weihnachtenund I have no idea how to verändern.wenn to the background on them, I preview an image and then the right is, there is no cause, “says set as Desktop Background” and If I set the right mouse button on a picture on the Internet, there is something that says, “as desktop background”, so I click it, but nothng passiert.Hilfe please:)