One of the changes to the Plasma workspace in Plasma 5 (based on KDE Frameworks 5) and continued in Plasma 6 (based on KDE Frameworks 6) is that the option of having separate Plasma widgets on each virtual desktop is no longer available. This is unfortunate, because one useful side effect of being able to do that was the ability to have different desktop backgrounds (wallpaper) on different virtual desktops, so proving a useful visual cue as to the desktop currently displayed.
The most relevant KDE bug is https://bugs.kde.org/show_bug.cgi?id=341143
The position of the Plasma developers, as stated in the evaluation of that bug, is that there have been major architectural changes within Plasma and at the moment the option is not available, with no simple way to bring it back. It has been acknowledged as an omission and is currently being worked on. Please be considerate to the developers and refrain from commenting on the bug unless you really do have a useful contribution to make to the discussion.
This system tray application attempts to being back the ability to have different wallpapers on different virtual desktops, pending the feature being reintroduced into Plasma. It allows a wallpaper image file to be configured for each virtual desktop, and will change the background when the virtual desktop changes. It does this by changing the wallpaper image settings, so there is some Plasma configuration needed in order to use it. See below for instructions on how to do that.
The application is built and installed in much the same way as any other KDE application. Assuming that you have Qt 6, KDE Frameworks 6 and the Plasma desktop installed, go to a suitable build location (e.g. your home directory) and do:
git clone https://github.com/martenjj/wallpaperswitch.git
cd wallpaperswitch
git switch kf6
mkdir build
cd build
cmake ..
make
sudo make install
Building requires CMake and also KDE Frameworks development headers and libraries. Depending on your distro, you may need to install additional packages for these. If you are running Kubuntu then see the README.kubuntu file for a list of packages required.
After the files are installed above, start the switcher either from whichever desktop launcher you use (it will appear in the "Utilities" category), or by typing the command 'wallpaperswitch' in a terminal or KRunner (summoned by typing Alt-F2). The "desktop" icon will appear in the system tray. If you set the "Start automatically on desktop startup" option on in the settings, the switcher will start automatically when the Plasma desktop starts.
If this is the first time that the application has been run the the configuration dialogue will appear automatically. Otherwise, click the right mouse button over the system tray icon and select the "Configure..." option from the menu. Turn on the "Enable the desktop wallpaper switcher" check box if necessary.
Each currently configured virtual desktop will be listed there. To select the wallpaper image for a desktop, double click the entry or select it and click the "Set Wallpaper" button. From the file dialogue that appears, choose an image file. Repeat the same for each virtual desktop or, if you have more than one screen, each screen for each virtual desktop.
Click the "Plasma desktop settings" link at the bottom right, and follow the instructions there to set the required desktop settings. Then finally click "OK" to save the configuration.
The wallpaper image that you selected for the current virtual desktop should now appear, and change when the virtual desktop changes.
Switching can be enabled or disabled using the "Enable Switching" option on the system tray popup menu.
If you are able to build and install the application, but it will not run or it is not able to change the desktop background, then try starting it from a terminal window within the Plasma desktop. Observe the debugging messages for anything about missing files or any other problems.
If this does not indicate the problem then please raise an issue in GitHub (http://github.com/martenjj/wallpaperswitch). Do not raise a bug against KDE Plasma.
The original wallpaper switch for Plasma 5 (branch 'master' in this repository) no longer works because of changes to the Plasma desktop slideshow plugin (commit ea32a761 to plasma-workspace).
If you are using Plasma 5 and wish to have different wallpaper images per virtual desktop, then I suggest that you install the "Vallpaper" plugin and set it as the desktop background. Its configuration dialogue will then allow you to select an image file (or multiple images) for each virtual desktop. You can obtain this plugin via GitHub, or by using the "Get New Plugins" button in the desktop configuration dialogue. I have no connection with Vallpaper, apart from being able to report that it appears to work well and does exactly what it says. Unfortunately it does not yet appear to have been updated for Plasma 6.
If you use virtual desktops and want wallpaper switching then I hope that this utility will come in useful, and also that one day it will not be necessary...
Jonathan Marten, http://github.com/martenjj