gistfile1.php

<?php while (have_posts()) : the_post(); $count++; ?>
    <?php $video = get_field('video'); ?>
    <a href="#" onclick="switchVideo(this, '<?php echo $video; ?>'); return false;" class="vidtn">
        <img src="<?php echo the_field('vidthumbnail'); ?>" />
    </a>
<?php endwhile; ?>

<script type="text/javascript">
    //
    // @param    original    the DOM element of the existing video to be replaced
    // @param    newVideo    the video that we are inserting in place of the original
    // @return   jQuery      a jQuery object containing the new video element
    //
    function switchVideo(original, newVideo) {
        var $newElem = $('<iframe width="853" height="480" src="http://www.youtube.com/embed/' + newVideo +
            '?rel=0" frameborder="0" allowfullscreen></iframe>');
        $newElem.replaceAll(original);
        return $newElem;
    }
</script>