The Clip property doesn't support percentages (yet). To achieve that effect, you can either:
1) Wrap the image in a div, set percentage width and height and overflow: hidden; For example:
<div id="test">
<img src="https://www.google.com/images/srpr/logo11w.png">
</div>
div#test {
width: 200px;
height: 100px;
overflow: hidden;
}
JSFiddle
2) Instead of the image, use a div with percentage width and height and the image set as a background-image.
3) Use javascript to calculate the required width and height after the image has been rendered and work with that.