Responsive Enhance
This script will check whether an image is larger than a specified width and load a full resolution image if necessary.
<img id="demo" src="http://dummyimage.com/400x300" alt="Responsive Image" data-fullsrc="http://dummyimage.com/800x600">
<script>responsiveEnhance(document.getElementById('demo'), 400);</script>
This will replace the image with the image defined in the data-fullsrc when the image is wider than 400 pixels.