border-radius

is your friend

i.e. http://jsfiddle.net/YEHdP/

Timmey's response should work. What browser are you using?
Border-Radius is a CSS3 feature, so it won't work on older browsers.

You can try giving the image a container with the size you want and give the container a radius as well:

HTML

<div>
<img src="http://placehold.it/100x100&quot; alt="" />
</div>

CSS

div {
	 width:100px;
	 height:100px;
	 overflow:hidden;
	 border-radius:50px;
}

link: http://jsfiddle.net/YEHdP/1/