<script>
function Go() {
document.getElementById('scr').src=(document.getElementById('typ').checked)? 'http://javascript.ru/forum/images/smilies/laugh.gif': 'http://javascript.ru/forum/images/smilies/blink.gif'
var n=(document.getElementById('typ').checked)? 500: 0
alert(n)
}
</script>
<img src="http://javascript.ru/forum/images/smilies/blink.gif" id="scr" /><br />
<input type="checkbox" name="check" id="typ" onclick="Go()" />