Sensing

<head>
  <respond>
    <case name="one"   media="min-width:200px"/>
    <case name="two"   media="min-width:400px"/>
    <case name="three" media="min-width:600px"/>
  </respond>
</head>

<body>
  <image alt="An alt tag for the image">
    <source src="mobile.jpg" case="one" />
    <source src="medium-res.jpg" case="two" />
    <source src="high-res.jpg" case="three" />
    <source src="very-high-res.jpg" media="min-width:800px" /><!-- special case not templated -->
    <img src="mobile.jpg" /><!-- fallback for old browsers -->
  </image>
</body>