I'm not sure they actually pass a value, as in value="$value", but they do pass the x and y coordinates of where the user clicked. Lets say you put two images in the same form named ImageSubmit1 and ImageSubmit2, you could check which one they clicked by using

if (defined($cgi->param('ImageSubmit1.x')) {
   #Do some code associated with ImageSubmit1
} elsif (defined($cgi->param('ImageSubmit2.x')) {
   #Do some code associated with ImageSubmit2
}
[download]

This way you can figure out what image the user clicked on.

Dave