<?php if(has_post_thumbnail( $the_ID) || $sl_thumb!=""){ ?>
  <?php
        if($sl_thumb!=""){
            echo "<a href='" . $url[0] . "'>";
            echo "<img src='" . $sl_thumb . "' alt='' title='#" . $sl_caption . "' />";
            echo "</a>";
        } else{
            echo "<a href='" . $url[0] . "'>";
            the_post_thumbnail( 'slider-post-thumbnail' );
            echo "</a>";
        }
        ?>