Trivera Retina Images adds support for retina images to the Concrete5 file manager. Upload your retina image and a version half the size will automatically be saved for non-retina displays with a file attribute that relates to the retina version. On the frontend if a retina display is detected the retina version will be displayed instead. This works within page content and blocks. See the documentation page for instructions on use.
What are retina images and why do I need them? http://www.sitepoint.com/support-retina-displays/
I quick way to test the package is right here on this page. If you are viewing it on a retina device then you will see the retina version of the image below. To switch between retina and non-retina views install the EditThisCookie http://www.editthiscookie.com/ browser extension and change the cookie value that is set when a retina display is detected. Using that extension or another like it change the devicePixelRatio cookie to a value of 2 or greater to emulate viewing on a retina device.
If everything is working you should see the 2x version of the image when the cookie value is greater than 2 and the non-retina version when the value is less than 2 when you refresh the page.