div#large_image_holder ul li {
    height: 320px;
    left: 10px;
    overflow: hidden;/* Remove this line */
    position: absolute; 
    top: 10px;
    width: 310px;
    z-index: 9;
}

div#large_image_holder {
    float: left;
    overflow: hidden;hidden;/* Remove this line */
    text-align: center;
    width: 100%;
}

<?php
// Showing The Additional Images
// if(!empty($this->product->images) && count($this->product->images)>1) {
if (!empty($this->product->images)) {
    ?>
    <div class="additional-images">
    	<ul id="thumb_holder">
			<?php
			// List all Images
			if (count($this->product->images) > 0) {
			    foreach ($this->product->images as $image) {
					//echo '<div class="floatleft">' . $image->displayMediaThumb('class="product-image"', true, 'class="modal"', true, true) . '</div>'; //'class="modal"'
					echo '<li><a href="javascript:void(0);"><img src="'.JURI::root().$image->file_url_thumb.'" alt="thumb" /></a></li>';
			    }
			}
			?>
		</ul>
        <div class="clear"></div>
    </div>
<?php