window.onload = initForms;

function initForms() {
  for (var i=0; i<document.forms.length; i++) {
    if (document.forms[i].name == "imageSearch") {
      document.forms[i].onsubmit = function() { return validForm(); }
    }
  }
}

function validForm() {
  var searchFor = document.getElementById("searchFor").value;

  if (searchFor == "") {
    alert("Please specify one or more search terms.");
    return false;
  } else {
    return true;
  }
}