function insertYourContent(){
    tinyMCE.activeEditor.execCommand('mceInsertContent',false,"Whatever text");
}
 
var myIcon = document.getElementById("sInput_netadvimage");
if(myIcon.attachEvent)
    myIcon.attachEvent("onclick",insertYourContent);
else if(myIcon.addEventListener)
    myIcon.addEventListener("click",insertYourContent,false);