Well, you can set the margin and padding of the body to zero or offset the the margin or position of the image to compensate for the body's default or set margin and padding. Since margins padding for the body differ from browser to browser, the best approach would be to either set them to zero or a size of your choice, then work with the properties of the image to position it where you like. One possibility:
<head>
<style type="text/css">
body {
margin:15px;
padding:0;
}
.flush {
margin-left:-15px;
}
</style>
</head>
<body>
<img class="flush" src="whatever.jpg">
</body>