Apple demoed macOS Mojave at the WWDC 2018 keynote yesterday and the new wallpapers are — as usual — fantastic. There are two this time, displaying a sand dune in the Mojave desert at two different points in time.
Since they’re designed to automatically switch depending on the time of day, this obviously won’t work under High Sierra or earlier. You can, however, create a folder with the Mojave wallpapers and set them to change automatically, every hour for example.
The header image is a compilation of the two wallpapers and compressed heavily for the web — please don’t use it, since it has a much lower resolution than the originals.