Thanks for looking at it!
These are the settings of the Random Image Module:
The exact path is not fully visible on the screenshot, but it's: /opt/joomla-3.6.2-1/apps/joomla/htdocs/images/a
And this is how the pictures are "displayed":
I wanted to put a second screenshot here, but it's not allowed for new users. Will post in second reply.
Note, that the file name of the picture to be displayed is found!
With exactly the same settings, everything had worked fine prior to changing the application directory to / (root).
Based on your screenshot, I've changed the image folder path from absolute to relative, meaning in my case: images/a.
And with this it works again!
I don't understand why doesn't the absolute path work after the app directory is changed to root, but I'll manage now and will use relative path.