there are many ways to change your image-background , i found a lot of scripts that do a lot of things and their use is complicated . one simple way is to use custom file-names and variables . this works pretty well , use it in your “header.php” theme file :
<?php // add your files ….
$bg_image=rand(1, 180);
echo “<style type=\”text/css\”> body { background: url(http://www.site.org/blog/wp-content/themes/images/load/custom”.$bg_image.”.jpg) no-repeat fixed center center #000; } </style> \n”
?>