i understand the one by default is made of 16 photos. Are they packed together in a special format ?
Are there any tools around to do our own dynamic wallpapers ?
You are correct, it is made up of 16 different photos saved inside Apple's new preferred image format, HEIC. But it's not just the images, there is metadata inside the HEIC file as well that tells the OS what images to show at what time based on GPS location.
There currently is no easy/WYSIWYG way to create your own dynamic wallpapers. You can manually create them, but it's a lot of work in the Terminal, etc.
If anyone is interested in this, some guy has put together a step-by-step guide here, but it still requires Terminal work.
