How do you stretch an image to fill a <div> while keeping the image's aspect-ratio?

Update 2016:

Modern browser behave much better. All you should need to do is to set the image width to 100% (demo)

.container img {
   width: 100%;
}

---

Since you don't know the aspect ratio, you'll have to use some scripting. Here is how I would do it with jQuery (demo):

CSS

.container {
    width: 40%;
    height: 40%;
    background: #444;
    margin: 0 auto;
}
.container img.wide {
    max-width: 100%;
    max-height: 100%;
    height: auto;
}
.container img.tall {
    max-height: 100%;
    max-width: 100%;
    width: auto;
}​

HTML

<div class="container">
 <img src="http://i48.tinypic.com/wrltuc.jpg" />
</div>
<br />
<br />
<div class="container">
 <img src="http://i47.tinypic.com/i1bek8.jpg" />
</div>

Script

$(window).load(function(){
 $('.container').find('img').each(function(){
  var imgClass = (this.width/this.height > 1) ? 'wide' : 'tall';
  $(this).addClass(imgClass);
 })
})

There is a much easier way to do this using only CSS and HTML:

HTML:

<div class="fill"></div>

CSS:

.fill {
    overflow: hidden;
    background-size: cover;
    background-position: center;
    background-image: url('path/to/image.jpg');
}

This will place your image as the background, and stretch it to fit the div size without distortion.

Not a perfect solution, but this CSS might help. The zoom is what makes this code work, and the factor should theoretically be infinite to work ideally for small images - but 2, 4, or 8 works fine in most cases.

#myImage {
    zoom: 2;  //increase if you have very small images

    display: block;
    margin: auto;

    height: auto;
    max-height: 100%;

    width: auto;
    max-width: 100%;
}