try this:
Code:
div.image {margin:2em auto}
div.image img {width:50%; max-width:100px}
<div class="image"><img alt="" src="images/ne-logo100.png"/></div>
As jhowell mentioned, the 50% is half your screen width (or half the width of the container)... the max-width tells it to NOT display larger than
the actual pixel size of the image 100px so it doesn't get overblown and pixelated.... that alt="images/ne-logo100.png" is actually worse than nothing at all... you can adjust the div.image settings to style it however you wish.