miew

<script type="text/javascript">
function background(){

    $("body")
    .css({"background-image":"none"})
    .css({"background-color":"green"});

}

function image(){
    $("body")
    .css({"background-color":"none"})
    .css({"background-image":"url(http://cdn.modernfarmer.com/wp-content/uploads/2014/03/smart_pig.jpg)"});
}
</script>

<html>
<body>
xxxxxx<br>
xxxxxx<br>
xxxxxx<br>
xxxxxx<br>

</body>
<input type=button value="background" onClick="background()" />
<input type=button value="image" onClick="image()" />
</html>


<script
  src="https://code.jquery.com/jquery-1.12.4.min.js"
  integrity="sha256-ZosEbRLbNQzLpnKIkEdrPv7lOy9C27hHQ+Xp8a4MxAQ="
  crossorigin="anonymous">
</script>