how to change image source to string in javascript code example

Example: js code to change a immage

<script type="text/javascript">
    function changeImage(a) {
        document.getElementById("img").src=a.src;
    }
</script>
<div id="main_img">
    <img id="img" src="1772031_29_b.jpg">
</div>
<div id="thumb_img">
    <img src='1772031_29_t.jpg'  onclick='changeImage("1772031_29_b.jpg");'>
    <img src='1772031_55_t.jpg'  onclick='changeImage("1772031_55_b.jpg");'>
    <img src='1772031_53_t.jpg'  onclick='changeImage("1772031_53_b.jpg");'>
</div>