Command line launch d8viewer to include subdirectories

I'm trying to configure d8viewer to display a digital picture frame slideshow at startup. I'm so close to getting it working but it seems d8viewer only displays images in the current folder. Is there a command line parameter that would instruct d8viewer to include subdirectories / subfolders?

Have a look here:

Very specific, thanks! Do you know if this can be invoked from command line? I'm trying to configure the system to start displaying images upon startup, without any user intervention.

Yes, with DOpusRT:

https://www.gpsoft.com.au/help/opus12/index.html#!Documents/DOpusRT_Reference.htm