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

Question

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%;
}

Update 2019.

You can now use the object-fit css property that accepts the following values: fill | contain | cover | none | scale-down

https://developer.mozilla.org/en-US/docs/Web/CSS/object-fit

As an example, you could have a container that holds an image:

<div class="container">
    <img src="" class="container_img" />
</div>

.container {
    height: 50px;
    width: 50%;
}

.container_img {
    height: 100%;
    width: 100%;
    object-fit: cover;
} 

If you can, use background images and set background-size: cover. This will make the background cover the whole element.

CSS

div {
  background-image: url(path/to/your/image.png);
  background-repeat: no-repeat;
  background-position: 50% 50%;
  background-size: cover;
}

If you're stuck with using inline images there are a few options. First, there is

object-fit

This property acts on images, videos and other objects similar to background-size: cover.

CSS

img {
  object-fit: cover;
}

Sadly, browser support is not that great with IE up to version 11 not supporting it at all. The next option uses jQuery

CSS + jQuery

HTML

<div>
  <img src="image.png" class="cover-image">
</div>

CSS

div {
  height: 8em;
  width: 15em;
}

Custom jQuery plugin

(function ($) {
  $.fn.coverImage = function(contain) {
    this.each(function() {
      var $this = $(this),
        src = $this.get(0).src,
        $wrapper = $this.parent();

      if (contain) {
        $wrapper.css({
          'background': 'url(' + src + ') 50% 50%/contain no-repeat'
        });
      } else {
        $wrapper.css({
          'background': 'url(' + src + ') 50% 50%/cover no-repeat'
        });
      }

      $this.remove();
    });

    return this;
  };
})(jQuery);

Use the plugin like this

jQuery('.cover-image').coverImage();

It will take an image, set it as a background image on the image's wrapper element and remove the img tag from the document. Lastly you could use

Pure CSS

You might use this as a fallback. The image will scale up to cover it's container but it won't scale down.

CSS

div {
  height: 8em;
  width: 15em;
  overflow: hidden;
}

div img {
  min-height: 100%;
  min-width: 100%;
  width: auto;
  height: auto;
  max-width: none;
  max-height: none;
  display: block;
  position: relative;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

Hope this might help somebody, happy coding!

Thanks to CSS3

img
{
   object-fit: contain;
}

https://developer.mozilla.org/en-US/docs/Web/CSS/object-fit

IE and EDGE as always outsiders: http://caniuse.com/#feat=object-fit