You are going to have to change the coding on your theme a bit to make this work, since its programmed to use bkgrnd_img X on this page and Y on another page. I think what it is, is you want to have a pool of bkgrnd images, any one of which can appear on any page.
I don’t have time to work the code out, but the approach I would take is generate a random # between 1 and 5
<?php $img = rand ( 1, 5 ); ?>
and on each page, assign a class to the DIV that holds the background image, like this (“hdr” I made up – you have to use the name that’s in your theme)
<div id="hdr" class="img<?php echo $img; ?>">
if the random number was 4, the output of that will be
<div id="hdr" class="img4">
Then in your CSS make an entry for images 1 – 5, assigning the background images like this:
#hdr.img4 { background-image: images/background_img4.jpg) no-repeat; }
So that image is associated with that class, and the class name is assigned in the HTML randomly from the PHP routine.
You have to remove the code from your theme’s header.php that assigns the images the way its done now.
make a copy of header.php before you do this in case it doesn’t work for you.