Hola
Peli. Proba con esto:
Código HTML:
<html>
<head>
<script languaje="javascript">
function caritas(Which){
document.form.comentario.value = document.form.comentario.value + Which;
}
</script>
</head>
<body>
<a href="javascript:caritas(' :D')"><img src="../smileys/smiley4.gif" border="0" /></a>
<form name="form" action="x" method="post">
<textarea name="comentario" id="comentario" rows="8" cols="38"></textarea>
</form>
</body>
</html>