Submit buttons as images

How do you determine which submit button was pressed if you are using images instead of standard buttons for form data submission. I am having a problem with a form I'm working on. Basically the user enters some information, reads an agreement (yeah right... lol) and either clicks Agree or Disagree. My problem is that when I got rid of the std html buttons in favor of a pair of images that went along with the site design better, I can no longer figure out for the life of me which button was pressed. I know in perl/cgi scripts you can access the x or y co-ords of the image to figure out which one was pressed, but trying flavors of this with PHP didn't work. Can anyone shed some light on this? Thanks in advance. Ben