If you know the aspect ratio of your images of the slider, there is a workaround.
Say your original image size is 1000 x 250 px, then you can create a DIV element with the { padding-top: 25%; height:0; width:100%; }, and it will have the same height as your slider. So you can make your slider has an absolute positioning, and put your slider inside the DIV element. For example:
<div style="position:relative;width:100%;padding-top:25%;>     
    <div id="ninja-slider" style="position:absolute;width:100%;left:0;top:0;">
        <ul>
            <li><div data-image="1.jpg"></div></li>
            <li><div data-image="2.jpg"></div></li>
            <li><div data-image="3.jpg"></div></li>
        </ul>
    </div>
</div>