Hmm...well, I can see what they mean. There really isn't any way to do what you want in the bbcode. You could do it in the template, but you'd have to find the php code that handles the template then make changes there. If the template is named vbpcsi_add_to_cart_link then somewhere there must be code that looks some thing like:
Code:
$template = vB_template::create('vbpcsi_add_to_cart_link');
$template->register('url', $url);
$something = $template->render();
and you'd have to add code to figure out what image you want and call $template->register() to add it to the template (and also add the vb:raw tag like I mentioned before).