There are two slightly different variations of this default image—one included in the initial developer beta and another in the final release—differing slightly in their crop and sky-to-mountain ratio. Common Sources for 1080p Versions
While 1920x1080 is a popular "Full HD" download size, the original asset was released at a high resolution of 5120 x 3684 pixels to support Retina displays.
The search term "1920x1080 Os X Sierra Wallpaper 38 Wallpaper Av..." appears to be a common search query string for finding high-definition desktop backgrounds originally included with . The Official macOS Sierra Wallpaper 1920x1080 Os X Sierra Wallpaper 38 Wallpaper Av...
Threads on subreddits like r/apple often share direct links to high-quality versions of every default Mac wallpaper.
Sites like WallpaperCave and 4KWallpapers offer the macOS Sierra mountain scene in 1920x1080, 4K, and 5K formats. There are two slightly different variations of this
If you are looking to download this specific resolution, several platforms host collections of Apple's stock wallpapers:
The primary wallpaper for macOS Sierra features a scenic view of the in California, specifically captured at dusk. The Official macOS Sierra Wallpaper Threads on subreddits
Lone Pine Peak in the Eastern Sierras, near the town of Lone Pine.