Hi Emiel,
That is not a link to a standalone experiment, it’s a link to an experiment on the PCIbex Farm. The resources (two images) seem to preload fine there
One thing that came to mind in the meantime: the exchange you referred to concerns running your study on a dedicated webserver. If you are running the experiment locally on your own device (eg. by typing python server.py in your terminal) then chances are you have no webserver running, just the server.py python script, which will not serve multimedia files from any folder. In that case, one solution would be to set up a XAMPP environment (or another webserver solution) so you can serve content at localhost:3000 (if you serve content elsewhere/on another port, then make sure to include the full URI to your files in your experiment)
Jeremy