This demo shows the image which Apple.com used for its iPad hero when the 3rd gen iPad launched. Its standard pixel size is 1454x605 (110KB filesize), whereas the high-resolution pixel size is 2908x1210 (352KB filesize). Its a huge friggin' picture, and you wouldn't want to download both of these when you'll only be viewing one of them.
If there is a red border on the top and bottom of the image then it is being viewed in standard-resolution. If the image has a green border on the top and bottom then it is being viewed in high-resolution.
This example does not do any server image resizing, but instead uses the data-src-high-resolution attribute to state which image src to use when this page is high-resolution enabled.
Photos courtesy of Apple.com
