The files do not end with a '~'.
So I got the file saved correctly, and after that the PiScreen backlight started working. The screen is black but backlit (so like dark grey). I finished the steps in the guide and ran the test. This appeared on my screen: https://i.imgur.com/BOdAERj.jpg (I see an error but don't know what it means) and on the Pi: https://i.imgur.com/JMXrIFr.jpg
After pressing enter on the screen, this was the output https://i.imgur.com/6rwvsIE.jpg
I'm not sure if this test succeeded since the image appeared on the display, or if it failed because there were errors.
I still can't get the desktop image to appear on the piscreen.