Can you apply a width to a :before/:after pseudo-element (content:url(image))?

This is in addition to my recent question: Is it possible to use pseudo-elements to make containing elements wrap around an absolutely-positioned element (like a clearfix)?

JSFiddle: http://jsfiddle.net/derekmx271/T7A7M/

I'm trying to use pseudo-elements to "clearfix" absolutely positioned images. I have gotten as far as getting the same image to display behind the slides, but cannot seem to apply widths to the inserted image. Am I crazy, or can you NOT apply widths to pseudo elements whose content is content: url(img/image.jpg)? I tried different variations of display:block, etc. to no avail...

#slider ul:after {
  content: url(http://www.cs7tutorials.com/img/slide1.jpg);
  display: block;
  position:relative;
  width:70px;
}

I need to set the width of the pseudo-element image to 100% and the max-width to 800px, so that it has the same dimensions as my slides.