node.replaceChild
<!DOCTYPE HTML>
<html>
<body>
<div style="background : url(/upload/41d.jpg);border-radius: 5px;width: 162px;height:207px;" alt="Эра" title="Эра" id="image_24"></div>
<script>
var div = document.getElementById('image_24');
var img = document.createElement('img');
img.setAttribute('style', div.getAttribute('style'));
img.setAttribute('alt', div.getAttribute('alt'));
img.title = div.title;
img.id = div.id;
div.parentNode.replaceChild(img, div);
</script>
</body>
</html>