Open up categories.php and find:

<code><a href="#" onclick="addNewPiForm();return false;"><span class="ui-icon ui-icon-plus" style="float: left;"></span><?php echo TEXT_PRODUCTS_ADD_LARGE_IMAGE; ?></a></code>

Change to:

<code><span class="tdbLink"><a href="#" id="add_image" onclick="addNewPiForm();return false;"><?php echo TEXT_PRODUCTS_ADD_LARGE_IMAGE; ?></a></span>
<script type="text/javascript">$("#add_image").button({icons:{primary:"ui-icon-plus"}}).addClass("ui-priority-secondary").parent().removeClass("tdbLink");</script></code>