onclick="getElementById('message').value = getElementById('message').value+':-)'"

function insert(textarea, form, avant, apres=null, inner=null){
  var champ = document.forms[form].elements[textarea];
  var textarea_value = champ.value;
 
  if(getSelection){
    champ.focus();
    var deb_sel = textarea_value.substr(0, champ.selectionStart);
    var end_sel = textarea_value.substr(champ.selectionEnd, textarea_value.length);
    var sel = getSelection();
    textarea_value = deb_sel + avant + sel + inner + apres + end_sel;
  }else if(champ.createTextRange){
    champ.focus();
    var sel = document.selection.createRange().text;
    document.selection.createRange().text = avant + sel + inner + apres;
  }
  champ.focus();
}
function smilies(form, textarea, smiley){
  insert(textarea, form, smiley);
}

<a href="javascript:void(0);return:false;" onClick="smilies('formulaire', 'message', ':)');return:false;"><img title=":)" alt=":)" src="url_du_smiley" /></a>
<!-- Avec un formulaire de ce type : -->
<form name="formulaire" action="page.html"method="post">
<label>Message :<br /><textarea name="message" rows="10" cols="25"></textarea></label><br />
<input type="submit" />
</form>