There's a guide for using polyfill.js (or picturefill) with magento here
After downloading picturefill.min.js, put it in one of your magento folders, for example skin/frontend/yourtheme/default/js
Add an entry to your app/design/frontend/yourtheme/default/layout/local.xml
file.
In the section
<layout>
<default>
<reference name="root">
<reference name="head">
Add
<action method="addItem"><type>skin_js</type><name>js/picturefill.min.js</name></action>
Then if you want to update your product list pages, edit app/design/frontend/yourtheme/default/template/catalog/product/list.phtml
. Find the img id
tags (will appear twice, once in list and grid views)
<img id="product-collection-image-<?php echo $_product->getId(); ?>"
srcset="<?php echo $this->helper('catalog/image')
->init($_product,'small_image')->keepFrame(FALSE)
->resize($_imgSize); ?> 1x,
<?php echo $this->helper('catalog/image')
->init($_product,'image')
->keepFrame(FALSE)
->resize(2*$_imgSize); ?> 2x"
The product detail page requires some other changes.
You can also use this magento picture-fill reference theme for this task.