<img id="id" src="http://javascript.ru/forum/images/reputation/reputation_plus.gif" alt="http://javascript.ru/forum/images/reputation/reputation_plus.gif" onclick="ChangeURLPicture(this)">
<script>
function ChangeURLPicture(a) {
var b=a.alt.replace("_plus","_minus");
if ( a.src == a.alt) { a.src = b;} else { a.src = a.alt;}
}
</script>
Выпендриваюсь ...