I just checked that demo. The image files are in the correct place for me but I can confirm that the demo isn’t set up correctly. The image size is specified as (0.5,0.5) but the units are in pixels, which will make the stimuli effectively invisible. The units setting needs to be height.
Thanks for bringing this to our attention. I’ve submitted a pull request here to address it:
but have no idea what was happening with your missing image stimulus files.