This is how simple it really can be:
$panel = rand(1,12);
"<div id=\"mainindex\" style=\"height: 264px; width:430px; background-image:url('login_bg".$panel.".jpg'); text-align:center;\">";
//End of code.
I took that directly from a page where I randomly have a different one of 12 images
as the background to the div "mainindex" every time a user opens the page.
The images are: login_bg1.jpg,login_bg2.jpg,login_bg3.jpg, etc... to... login_bg12.jpg
The images are in the same directory as my page file so I don't have to do a full path.
Php is a beautiful thing.