Put your images in a folder called 'random' in your ROOT directory.

Please this script at the top of your page:
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<?php function RandomFile($folder='', $extensions='.*'){
   // fix path:
    $folder = trim($folder);
    $folder = ($folder == '') ? './' : $folder;
 
    // check folder:
    if (!is_dir($folder)){ die('invalid folder given!'); }
 
    // create files array
    $files = array();
 
    // open directory
    if ($dir = @opendir($folder)){
 
        // go trough all files:
        while($file = readdir($dir)){
 
            if (!preg_match('/^\.+$/', $file) and 
                preg_match('/\.('.$extensions.')$/', $file)){
 
                // feed the array:
                $files[] = $file;                
            }            
        }        
        // close directory
        closedir($dir);    
    }
    else {
        die('Could not open the folder "'.$folder.'"');
    }
 
    if (count($files) == 0){
        die('No files where found :-(');
    }
 
    // seed random function:
    mt_srand((double)microtime()*1000000);
 
    // get an random index:
    $rand = mt_rand(0, count($files)-1);
 
    // check again:
    if (!isset($files[$rand])){
        die('Array index was not found! very strange!');
    }
 
    // return the random file:
    return $folder . "/" . $files[$rand];
 
}


$random1 = RandomFile("random");
while (!$random2 || $random2 == $random1) {
	$random2 = RandomFile("random");
}
while (!$random3 || $random3 == $random1 || $random3 == $random2) {
	$random3 = RandomFile("random");
}
while (!$random4 || $random4 == $random1 || $random4 == $random2 || $random4 == $random3) {
	$random4 = RandomFile("random");
}
?>


And place this section where you'd like to display the images:
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<div id="random_images">
	<img src="/<?php echo $random1; ?>" alt="image alt" />
	<img src="/<?php echo $random2; ?>" alt="image alt" />
	<img src="/<?php echo $random3; ?>" alt="image alt" />
	<img src="/<?php echo $random4; ?>" alt="image alt" />
   </div>