<body style="background: url(img/123.jpg)">
<button data-src="img/1234.jpg"></button>
</body>$('button').on('click', function(){
var src = $(this).attr('data-src');
$('body').css({background: 'url('+src+')'})
});.outer {
display: block;
width: 100%;
}
.inner {
display: block
width: 70%
margin: 0 auto;
}<div class="outer">
<div class="inner">
</div>
</div>