The only place we mention the background_image parameter is here: https://www.cendio.com/resources/docs/tag-devel/html/ch14s02.html#configuration_sessionstart And we don't mention that the program called 'xsri' is required for this function to work. The only hint is a message in the xinit.log, not even a warning.
We have moved away from using xsri as our way of setting a background image since it is quite deprecated nowadays, so this bug is no longer relevant.