background image settings

Hello ! I'm trying to setup my self-compiled copy of TDE. I opened "Trinity Control Center" and then "Appearance & Themes" and then "Background". There I selected a picture "Aghi" (showing some green tree branches) and pressed "Apply" (then spent like 10 mins for similar steps, never mind these) As you can see on the screenshot https://i.paste.pics/7kbai.png the background is not changed at all, it's some kind of default picture being there from the very beginning. On my real computer with TDE from official repository there is no such problem, so I suppose I might have either installed or started TDE in some wrong way on this hacking clean system (which is running inside a docker container, so not even systemd is present there). My question is: what process/daemon can be missed or maybe some other reason for this wrong background behaviour ? Just to explain why I so much care about background on a development system: it's about bug #3059 (X11 resources leak). After some research I'm almost sure it's about slide show on background window and now I just want to reproduce this bug in "clean lab conditions". Cheers, Ilya D.