/* Scandir */

$thumbsDesignPath = "trabalhos/design/thumbs";
$imagesDesignPath = "trabalhos/design/large";
$file_display = array('jpg', 'jpeg', 'png', 'gif');

if (file_exists($thumbsDesignPath) == false){
    echo 'Directory \'', $thumbsDesignPath, '\' not found.';
} else {
    if (file_exists($imagesDesignPath) == false){
    echo 'Directory \'', $imagesDesignPath, '\' not found.';
    } else {
        $thumbs_content = scandir($thumbsDesignPath);
        $images_content = scandir($imagesDesignPath);
        $design_content = $thumbs_content + $images_content;
        echo '<pre>'; print_r($design_content); echo '</pre>';
        $counter = 0;
            $filesPerPage = 6;
                foreach ($design_content as $design_file)
                {
                $design_file_type = (end(explode('.', $design_file)));
                    if($design_file !== '.' && $design_file !== '..' && in_array($design_file_type, $file_display) == true){
                                if($counter == 0){
                                    echo "<li>";
                                    $counter++;
                                    /*$info = pathinfo($file);
                                    $sFile =  basename($file,'.'.$info['extension']);

                                    $aFiles = glob($images.$sFile."[0-9]+.[".implode($file_display,"|")."]"); //This array shows all images relative to the thumbnail that you are seeing*/
                                    echo '<div class="portfolio_pic_hold"> <a href="'.$imagesDesignPath. '/' .$design_file.'" rel="lightbox[Design]"> <img src="'.$thumbsDesignPath. '/'.$design_file. '" /> </a> </div>';

                                    if($counter == $filesPerPage){
                                    echo "</li>";
                                    }
                                }
                        }
                }
        }

    }

Result:

Array
(
    [0] => .
    [1] => ..
    [2] => pinheirinhonovo.jpg
    [3] => qclass.jpg
    [4] => sylviartz.jpg
    [5] => teresabrito.jpg
    [6] => tgi.jpg
    [7] => valoresremodelados.jpg
    [8] => videoclubebaviera.jpg
    [9] => xtremenutrition.jpg
)

/* Glob */
$thumbsDesignPath = "trabalhos/design/thumbs/";
$imagesDesignPath = "trabalhos/design/large/";
$file_display = array('jpg', 'jpeg', 'png', 'gif');


if (file_exists($thumbsDesignPath) == false)
{
    echo 'Directory \'', $thumbsDesignPath, '\' not found.';
}
else
{
        $thumbs = glob($thumbsDesignPath."*.[".implode($file_display,"|")."]");
        print_r($thumbs);
        if (file_exists($imagesDesignPath) == false){
                echo 'Directory \'', $imagesDesignPath, '\' not found.';
        } else
        {
                $counter = 0;
                $filesPerPage = 6;
                foreach ($thumbs as $file)
                {

                        if($counter == 0) echo "<li>";
                        $counter++;
                        $info = pathinfo($file);
                        $sFile =  basename($file,'.'.$info['extension']);

                        $aFiles = glob($images.$sFile."[0-9]+.[".implode($file_display,"|")."]"); //Este array tem todas as imagens relativas ao thumbnail que estás a ver
                        echo '<div class="portfolio_pic_hold"><a href="'. $imagesDesignPath. '/'. $file.'"> <img src="'. $thumbsDesignPath . '/' . $file . '" /> </a></div>';
                        if($counter == $filesPerPage) echo "</li>";
                }
        }
}

Result:

Array
(
)