how to change image source using javascript

1//change image src
2document.getElementById('myImageID').src="images/my_other_image.png";

1document.getElementById("myImg").src = "hackanm.gif";

1<script type="text/javascript">
2    function changeImage(a) {
3        document.getElementById("img").src=a.src;
4    }
5</script>
6<div id="main_img">
7    <img id="img" src="1772031_29_b.jpg">
8</div>
9<div id="thumb_img">
10    <img src='1772031_29_t.jpg'  onclick='changeImage("1772031_29_b.jpg");'>
11    <img src='1772031_55_t.jpg'  onclick='changeImage("1772031_55_b.jpg");'>
12    <img src='1772031_53_t.jpg'  onclick='changeImage("1772031_53_b.jpg");'>
13</div>
14