AFAIR the submit-value of an input type="image" append the coordinates of the mouse-click on the given image (or maybe browser). However, the script that captures the form gets something like

  btLogin.X => $numeric,
  btLogin.Y => $numeric,

To be sure just copy the HTML-Code from the site, change the action location into a location of a small CGI script that captures all submitted values. It may be as small as the following:

use CGI;
use Data::Dumper;
my $q = new CGI;
my %vars = $q->Vars();
print "Content-Type: text/plain\n\n";
print Dumper(\%vars);
[download]

After submitting your copied form you'll see exactly the same parameters the ASP-site gets from the original site.