An easy to use jQuery plugin that allows you to target images and have them expand on an event. Follow the link to see it in action ActiveImage.
Just add the 'active-image.js' file to your project:
<script src="your-project/active-image.js"></script>
And call the plugin:
$( 'img' ).activeImage();
Thats it! The plugin handles the math so that your image retains its aspec ratio and fits within the viewport.