this should meet what's required
and can be quickly edited to handle a $fileType array

// Put all files of a directory of A selected filetype into an array.

function dir2array($targetDir, $fileType){
    $filesArray = array();
    if ($handle = opendir($targetDir)){
        while (false !== ($file = readdir($handle))){
            if(end(explode(".", $file)) == $fileType){
                $filesArray[] = $file;
                }
            }
            closedir($handle);
        }
    if (count($filesArray) > 0) {
        return $filesArray;
    } else {
        return FALSE;
        }
    }

// Usage examples:

$arrayName = dir2array("path/to/directory", "jpg");
$randfile = $arrayName[array_rand($arrayName)];

if(FALSE !== ($arrayName = dir2array("path/to/directory", "png"))){ $randfile = $arrayName[array_rand($arrayName)]; }