смотря какая ширина ...
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
img{
width: 50px;
}
</style>
<script>
window.onload = function(){
var i = document.getElementById('avatar-image-container').getElementsByTagName('img')[0];
alert(i.width);
var src = i.src;
b = new Image;
b.src = src;
alert(b.width);
};
</script>
</head>
<body>
<div class="avatar">
<div class="progress-bar hidden">
<div id="avatar-image-container" style="margin-left: -36px">
<img src="http://javascript.ru/forum/images/ca_serenity/misc/logo.gif" alt="">
</div>
</div>
</div>
</body>
</html>