Want to images load first then title in WordPress loop

Try it

<div class="main-interior portfolio" id="portfolio-big-pics" style="display: block;">
<?php
$args = array( 'post_type' => 'portfolio', 'order' => 'ASC');
$loop = new WP_Query( $args );
$img_titles="";
while ( $loop->have_posts() ) : $loop->the_post();
    $extraLastClass = $loop->current_post + 1 === $loop->post_count ? ' main-image-porfolio-main' : '';
    the_post_thumbnail( "thumbnail", array( "class" => "main-image portfolio $extraLastClass" ) );

    $img_titles .= '<div class="portfolio-box">

                <h5>Portfolio</h5>
                <ul class="item-list" id="portfolio-list">
                <li><a href="'.get_permalink().'">'.get_the_title().'</a>
                </li>               
                </ul>

            </div>';

 endwhile;
 ?>            
</div>
<?php echo $img_titles; ?>