Random Full-Size BG Image Code for plugin creation

I asked this on StackOverflow as well and haven’t received an answer.

I’m having an issue creating a full screen background for my WordPress theme that uses random images from the image library. I want to write a PHP function that can be used on multiple sites, so I can’t simply use the direct path in the code. It also needs to work on MultiSite in such a way that it only pulls images that are uploaded to that site. Here’s the code I’m working with:
HTML for my Background Div

<div id="background" class="background" style="background-image:url(<?php displayBackground();?>);">
</div>

PHP to randomize my image folder

<? php
function displayBackground()
{
$uploads = wp_upload_dir();
$img_dir = ( $uploads['baseurl'] . $uploads['subdir'] );
$cnt = 0;
$bgArray= array();

        /*if we can load the directory*/
if ($handle = opendir($img_dir)) {

    /* Loop through the directory here */
    while (false !== ($entry = readdir($handle))) {

    $pathToFile = $img_dir.$entry;
    if(is_file($pathToFile)) //if the files exists
    {   

        //make sure the file is an image...there might be a better way to do this
        if(getimagesize($pathToFile)!=FALSE)
        {
            //add it to the array
            $bgArray[$cnt]= $pathToFile;
            $cnt = $cnt+1;

        }

    }   

}
//create a random number, then use the image whos key matches the number
$myRand = rand(0,($cnt-1));
$val = $bgArray[$myRand];

}
closedir($handle);
echo('"'.$val.'"');

}

I know that my CSS markup is correct because if I give the DIV a fixed image location, I get a fullscreen image. Can anyone tell me what to do to fix it?

Also, I’m doing it this way because I don’t like the way the WPSupersize.js script affects the site styling.