why does this code not work?

[nazca]

<?php
//need to drop an add to cart button here.

echo zen_draw_form('cart_quantity', zen_href_link(zen_get_info_page($products_id), zen_get_all_get_params(array('action')) . 'action=add_product', $request_type), 'post', 'enctype="multipart/form-data"');
?>
<input type="hidden" name="cart_quantity" value="<?php echo $qty ; ?>" maxlength="6" size="4" />
<input type="hidden" name="products_id" value="<?php echo $products_id ; ?>" />
<input type="image" name="image" title=" Add to Cart " src="button_in_cart.gif" alt="Add to Cart " />
</form>

Using this form the item adds to cart successfully but never with the correct quantity. What do I need to change to get to the correct quantity?

EDIT: Changed the img src url for a little more security.