Background images should be able to look different ways, for example, "fill the entire window (scaled", "center the image (centered)" or "repeat the image on the entire window area (repeat)". Reproducible: Always
Going further with the request, maybe it would be nice if the was some kind of control on where exactly the image should appear (upper left corner, down right corner, upper right corner, etc...)
*** Bug 403556 has been marked as a duplicate of this bug. ***
This is still current (with KDE Plasma Version: 5.14.5 ; KDE Frameworks Version: 5.55.0 )