How to stretch the background image to fill a div

For this you can use CSS3 background-size property. Write like this:

#div2{
    background-image:url(http://s7.static.hootsuite.com/3-0-48/images/themes/classic/streams/message-gradient.png);
    -moz-background-size:100% 100%;
    -webkit-background-size:100% 100%;
    background-size:100% 100%;
    height:180px;
    width:200px;
    border: 1px solid red;
}

Check this: http://jsfiddle.net/qdzaw/1/

Modern CSS3 (recommended for the future & probably the best solution)

.selector{
   background-size: cover;
   /* stretches background WITHOUT deformation so it would fill the background space,
      it may crop the image if the image's dimensions are in different ratio,
      than the element dimensions. */
}

Max. stretch without crop nor deformation (may not fill the background): background-size: contain;
Force absolute stretch (may cause deformation, but no crop): background-size: 100% 100%;

"Old" CSS "always working" way

Absolute positioning image as a first child of the (relative positioned) parent and stretching it to the parent size.

HTML

<div class="selector">
   <img src="path.extension" alt="alt text">
   <!-- some other content -->
</div>

Equivalent of CSS3 background-size: cover; :

To achieve this dynamically, you would have to use the opposite of contain method alternative (see below) and if you need to center the cropped image, you would need a JavaScript to do that dynamically - e.g. using jQuery:

$('.selector img').each(function(){ 
   $(this).css({ 
      "left": "50%", 
      "margin-left": "-"+( $(this).width()/2 )+"px", 
      "top": "50%", 
      "margin-top": "-"+( $(this).height()/2 )+"px" 
   }); 
});

Practical example:

Equivalent of CSS3 background-size: contain; :

This one can be a bit tricky - the dimension of your background that would overflow the parent will have CSS set to 100% the other one to auto. Practical example:

.selector img{
   position: absolute; top:0; left: 0;
   width: 100%;
   height: auto;
   /* -- OR -- */
   /* width: auto; 
      height: 100%; */
}

Equivalent of CSS3 background-size: 100% 100%; :

.selector img{
   position: absolute; top:0; left: 0;
   width: 100%;
   height: 100%;
}

PS: To do the equivalents of cover/contain in the "old" way completely dynamically (so you will not have to care about overflows/ratios) you would have to use javascript to detect the ratios for you and set the dimensions as described...

Add

background-size:100% 100%;

to your css underneath background-image.

You can also specify exact dimensions, i.e.:

background-size: 30px 40px;

Here: JSFiddle

You can use:

background-size: cover;

Or just use a big background image with:

background: url('../images/teaser.jpg') no-repeat center #eee;