First, create a container <div> with your new height and width specified. Make sure that overflow is hidden. Next, place your image in the middle with the full dimensions specified, but also with a negative margin-left and margin-top. If you're wanting to center the cropping, both of those should be exactly half the number of pixels you're wanting to reduce the height or width by.
That sounded more complicated than it actually is. Here, take a look at this example instead:
1. <div style="width:51px;height:133px;overflow:hidden;"> 2. <img src="../images/inkplant/code/penguin.jpg" style="width:111px;height:193px;margin-left:-30px;margin-top:-30px;" alt="Cropped Penguin" /> 3. </div>
And here's the resulting penguin, or at least the parts of him that we've left:
Don't worry, no real penguins were harmed in the making of this post.