Thanks for merging my question in here, there's some useful information. It's still not clear to me what the "hidden" placement is for, though. Setting the placement to "hidden" and setting the image using CSS leaves me with no feedback tab. The way to use my own image appears to be to do what Matt M. suggests above and leave the placement as something like "left" or "right", and then to override the a#fdbk_tab's background-image using CSS (which has to be done *after* the widget's script). But what I'm trying to do is pop up the overlay from a page element that I have control over, so that I can work it into my page design.
So I still don't understand what the "hidden" placement is there for, and I still don't know how to use one of my own page elements -- links, buttons, whatever -- pop up the feedback overlay.
So I still don't understand what the "hidden" placement is there for, and I still don't know how to use one of my own page elements -- links, buttons, whatever -- pop up the feedback overlay.