It's almost a tradition that Apple introduces new wallpapers with every major software release. Even iOS 12 and macOS Mojave are no exception. While only a new wallpaper with a typical design was introduced for iOS 12, Mojave offers a small selection. Apple announced that the new Mac operating system will support dynamic wallpapers. These then change depending on the time of day.
You can download the new wallpaper from iOS 12 here . There is a version for the iPhone and an iPad version available.
You can join Deviant Art for the Mojave wallpaper. There the user "specialized666" has provided all four variants of the screen background. The zip file also has different resolutions from 1920x1080 pixels to 5120x2880 pixels, so you're sure to find the right resolution for your Mac.