LWP and <input type=image...>

I'm attempting to automate a form submission that uses a series of
<input type=image ...> tags, but I can't seem to figure out what I
need to post back to the server.  If I watch the POST request using
LiveHTTPHeaders, I see the x & y coordinates being posted (as expected
from the spec), but I'm not sure how the event/form handler on the
server interprets it, nor how I would simulate that behavior using
LWP.

Has anyone dealt with this before?  I'm at a loss after Googling and
reading up on how the ASP.NET postback handler works it's magic on
these types of forms, so any help is appreciated.

Thanks,
Chris