JavaScript onclick inquiry

I have a form with a checkbox that must be clicked before the form can be processed.  My code for the checkbox looks like this:

<input type="checkbox" name="iagree" id="iagree" onclick="clickIagree()"> I agree

and clickIagree() looks like this:

function clickIagree() {
      if (document.signupform.iagree.checked==true) {
            document.signupform.signup.disabled = false;
      } else {
            document.signupform.signup.disabled = true;            
      }
}

My error debug message is saying document.signupform.signup has no properties. Although I do have a signup form:

<form name="signupform" method="POST" action="thank_you.php" >

Perhaps it is my input image?

<input type="image" src="img/push-signup-small.png" style="margin: 0; padding: 0; cursor: pointer;" disabled onclick="verifyinput()" id="signup" name="signup">

Thanks.