var clickmessage="Copyright!"

   function disableclick(e) {
    if (document.all) {
      if (event.button==2||event.button==3) {
         if (event.srcElement.tagName=="IMG"){
            alert(clickmessage);
            return false;
         }
      }
    }
    else if (document.layers) {
       if (e.which == 3) {
          alert(clickmessage);
          return false;
       }
    }
    else if (document.getElementById){
       if (e.which==3&&e.target.tagName=="IMG"){
          alert(clickmessage)
          return false
       }
    }
   }

function associateimages(){
for(i=0;i<document.images.length;i++)
document.images[i].onmousedown=disableclick;
}

if (document.all)
document.onmousedown=disableclick
else if (document.getElementById)
document.onmouseup=disableclick
else if (document.layers)
associateimages()