Although the wallpaper from Apple’s iMac webpage is not populated in the current Yosemite build, it may only appear for Apple iMac 5K Display models.
In an effort to release this incredible image to the masses JasonZigrino worked a little magic to pull this official wallpaper download directly from Apple. He reformatted the 5K wallpaper across all devices and their native resolutions.
iOS: iPad; iPhone 5/s; iPhone 6/ 6 Plus
Desktop: 1440 x 900; 2560 x 1440; 2880 x 1800; true 5K image at 5241 x 2948