In theory, this is exactly what the clip
property of CSS is for - but there's one, sometimes really painful, side effect to using it, though - the image needs to be absolutely positioned:
<html>
<head>
<style type="text/css">
.thumbnail {
width:200px;
height:150px;
}
.thumbnail img {
position:absolute;
clip:rect(0, 200px, 150px, 0);
}
</style>
</head>
<body>
<div class="thumbnail"><img src="http://uhaweb.hartford.edu/SDUNN/sandwich.jpg"></div>
<div class="thumbnail"><img src="http://uhaweb.hartford.edu/SDUNN/sandwich.jpg"></div>
<div class="thumbnail"><img src="http://uhaweb.hartford.edu/SDUNN/sandwich.jpg"></div>
<div class="thumbnail"><img src="http://uhaweb.hartford.edu/SDUNN/sandwich.jpg"></div>
</body>
</html>
The fact that this takes the images out of document flow is pretty nasty - the best you can do is put them inside a frame of the right dimensions (which means you may as well just use the overflow mask methods other people have suggested). Clip is a useful property in the right places, and a lot of people don't seem to know about it.