As others have indicated, there are many ways to do this. The A element isn't an anchor, it's a link. And no one really uses XHTML on the web so get rid of the XML-style syntax.
If you don't have an id for the image, then consider:
function changeImage(id, src) {
var image;
var el = document.getElementById(id);
if (el) {
image = el.getElementsByTagName('img')[0];
if (image) {
image.src = src;
}
}
}
Then you can use an onload listener:
or add a script element after the link (say just before the closing body tag) or use some other strategy for running the function after the image is in the document.