I am not going to test this one, but it looks like any image with a link that references:
"javascript
: return !popupOpen(xcart_web_dir + '/popup_ask.php?productid=`$product.productid`')"
is going to work.
So a standard setup for a linked image should be fine.
ie:
<a name="mypopuplink" id="mypopuplink" href="javascript
: return !popupOpen(xcart_web_dir + '/popup_ask.php?productid=`$product.productid`')"><i mg src="{ImagesDir}/mypopupimage.jpg" /></a>
Then upload your image to skins/common_files/images or skins/images on older x-carts.
The name and id attributes I defined may not be necessary, but it is a good practice to always assign something.