Unable to set Background Images outside chilli theme directory

[Chronosaurus42]

Hello,
it is not possible to use background images if the file for it is outside the chilli theme directory.
This is possible with other themes e.g. Vortex-SDDM, but I want to get rid of the KDE dependency.

It is possible to get the directory with the desired background images via a "bind"-mount into the themes directory, but this is a dirty workaround.

Is there a technical reason why it is possible with some themes and not with others?
And more important it can be fixed?