Recently I've updated two laptops, that originally had Windows 8 installed, first to 8.1 and then to Windows 10. Each of them has shown the same sequence of wallpaper images (located at C:\Windows\Web\Wallpaper\Windows 10) and then got permanently stuck on the last image. The images in the above listed folder are labeled img1.jpg, img2.jpg, img3.jpg, img4.jpg and img13.jpg, the last being a vertically truncated version of img4. The image that remains on the screen appears to be img4, a post-sunset wilderness nighttime campground scene with an illuminated tent and what appears to be a slightly overdone Milky Way above it.
Right-clicking anywhere on the screen produces a menu, among whose offerings are "Next desktop background" - but once you have reached img4, selecting "next desktop background" has no effect, leaving the tent scene unchanged.
My desktop machine was upgraded directly from Windows 7 Professional. I decided immediately I did not like the Windows 10 wallpapers and replaced them with a set of wallpaper images taken from Windows 7 (which in this case was still on the hard drive under \windows.old\. There were several folders with images that could be selected under Windows 7. I copied several of those folders (the ones I liked) into a folder "Wallpaper" under "Pictures" (though I could have put it anywhere) and selected my favorite one as a "Slideshow". I presume I can do the same on the laptops, putting into those folders any images I like. It's not quite the same as creating themes - for instance, without a bit more work it doesn't come with custom sound effects - but it does give me eye-pleasing wallpaper and that was what I was after. I can select any of those folders under \Pictures\Wallpaper\ and get a revolving display of whatever images I choose to put into the selected folder. You can set the duration of each image (how often they change), the way it is displayed, and so on.
Right-click on the screen; select "Personalize", select "Background", select "Slideshow" and pick the folder in which are stored the images you want. Not quite "themes" but as they say, good enough for Government work <G>.
So I know I can put up a rotation of any pictures I want (even including the ones in the original Windows 10 wallpaper folder, perhaps without img13). My question here is a different one: can anybody figure out why the images in the original Win 10 wallpaper folder don't continue to rotate round and round the sequence, but stop at the end of the only 4 images in that folder? It strikes me as very strange.
Actually if you dig a bit deeper into Windows 10 it starts to look even stranger. Under "Wallpaper" there is a whole series of folders, many with the same names as the wallpaper folders in Windows 7, but most of them empty. The one called "Windows 10" that I've already described looks as if it might be called "outdoor recreation" or something like that - a girl running on a beach, someone swimming with fins, an extreme rockclimber on an impressive cliff face, and a tent in the wilderness . . . Now Windows 10, when it starts up, fairly bombards the user with striking images. A few of those appear in a folder called "\Screen\" but most of them do not (too bad, I'd like to put some of them into my slideshow!). Is this a project somebody started, and then got sidetracked before it was finished?
Being too curious for my own good, I'll probably grab some of the Win 7 images, put them into the like-named folders in \Web\Wallpaper\ in Win 10, and see what happens if I select one of those folders not as a "slideshow" but as a theme? Maybe it would need a custom win.ini file for each folder? As you can see, I am fumbling around, don't quite have all this in my grasp yet.