As we know it been a year since WordPress plugin directory support for header image as Matt mentioned an update last year. Since Apple came up with Retina Display / HiDPI devices which is basically double the pixels, WordPress is already support it, as well as the wordpress.org and WordPress Plugin Directory.

Simply you just create a double size of header image which is 1544×500 pixels named banner-1544×500.(jpg|png), then add to SVN assets directory along with banner-772×250.(jpg|png) and let WordPress Plugin Directory do the rest.

If you got Retina Display / HiDPI device in your hand, see it in action, check out JetPack or my Currencyr, there will be a shiny header image on the page.