Additionally, the Highlands map is referred to as "Training Preserve" on the Flickr account where the images are hosted, so it may not be related to the old High Ground map at all. Oddly enough, Unearthed bears the strongest resemblance to High Ground, but, unfortunately, the low resolution of the images makes detailed analysis difficult.
As of publishing, the Halo Destiny site is down for maintenance, though the Flickr images are still available. We've contacted Bungie for comment and will update if and when we learn more.
Update: Halo Destiny has posted a message noting that the site is temporarily offline following a recent request from Microsoft and Bungie to remove the Defiant Map Pack post. The Flickr images remain available as of this update.
[Thanks to everyone who sent this in!]