• Lightweight
• No dependencies
• Requires support for the Selectors API (but could easily be rewritten not to)

Add the data-imgsrc attribute to a placeholder element (such as <div>). It's value should contain a URL to the image you wish to conditionally load. Classes added to this placeholder will be carried over to the image. For example:

<div class="pull-left" data-imgsrc="/path/to/image.jpg">Fallback content</div>

Should the conditions for enhanced images be met, this element will be replaced with an image, as follows:

<img class="pull-left" src="/path/to/image.jpg"/>

If your markup contains thumbnails that link through to higher resolution images (say within a photo gallery or carousel that is progressively enhanced), you can simply add a valueless data-imgsrc attribute to each link. The value of the href attribute will be utilised by this script. For example, the following linked thumbnail:

<a class="pull-left" data-imgsrc href="/path/to/image_hires.jpg" title="View larger version of this photo">
	<img src="/path/to/image_lowres.jpg" width="360" alt=""/>
</a>

will be replaced as follows:

<img class="pull-left" class="pull-left" src="/path/to/image_hires.jpg"/>

Again, any classes added to this link will be carried over to new image.

Once you have added the data-imgsrc attribute to placeholder elements and any linked thumbnails, call the loadImgs() function at the end of your document:

<script src="data-imgsrc.js"></script>
<script>
	loadImgs();
</script>

Of course, this script is much more useful when combined with testing for certain device properties. For example, if you wanted to load enhanced images only when the viewport is wider than 40em (640px), you could do so as follows:

<script src="data-imgsrc.js"></script>
<script>
	if (window.matchMedia && window.matchMedia("(min-width: 40em)").matches) {
		loadImgs();
	}
<script>

Alternatively, you might wish to load image enhancements above certain bandwidth criteria (i.e. for agents that support the Network Information API):

<script src="data-imgsrc.js"></script>
<script>
	if (navigator.connection && navigator.connection.bandwidth >= 750) {
		 loadImgs();
	}
<script>

Examples of this script being used to enhance common design patterns can be found here: http://paulrobertlloyd.github.com/responsivepatterns/

Concept: Paul Robert Lloyd (http://paulrobertlloyd.com/)
Initial code: Josh Emerson (http://joshemerson.co.uk/)

This work is made available under the MIT license: http://paulrobertlloyd.mit-license.org/