Thank you both for the help!
Surely I should have read the wiki article about LightDM more carefully before I posted this question. Let me briefly summarize what was actually going wrong and how it was (temporarily) fixed.
I have to admit that I am still not 100% clear about every details, but at least I know more than before. And here is what I know so far:
- I edited the wrong file.
I think that was the reason why it didn’t work even if I configured using the non-GUI way.
The images type and size
While jpg images seem to be working just as well as png, the image should be resized to the exact dimension that fits the theme.
As far as I can see, the GUI setting tool should do exactly the same thing as /etc/lightdm/lightdm-gtk-greeter.conf, which is logical. But even if it was run as root, the images just won’t show up.
Yes, I switched to lightdm-gtk+ because it has a quick and lazy gui tool. In my other computer, which also runs Antergos, but with Gnome instead of xfce, the same gui tool is working perfectly. It does even auto-scale the images.
More comments and suggestions are welcome:)