In Kwin’s cube effect, you can set a background image. This has worked until somewhere in 2020, but in recent Plasma releases, it is broken. You only get the black background no matter what image is set. Observed on X.org. Linux/KDE Plasma: Arch Linux KDE Plasma Version: 5.22.4 KDE Frameworks Version: 5.85.0 Qt Version: 5.15.2
*** This bug has been marked as a duplicate of bug 430874 ***