<?php
$image = ['image path', 'image path', 'image path'];
?>
html
{
background: url(<?php echo $backgroundImage = $image[mt_rand(0, count($image) - 1)]; ?>) no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
overflow-x: hidden;
}
Generate random images with php and css