There are 3 tutorials in https://developer.puri.sm/Librem5/Apps/Tutorials.
I needed much time, to learn the tutorial “Application Resources”. Then I tried to use also a text file instead of a picture as Resource file. But I found no function like Gtk.Image.new_from_resource() to open and use the text file. After a lot of searching I found:
Warning In GTK+ 3.0, resource files have been deprecated …
Maybe the tutorial should be exchanged, with a more useful tutorial. But I am only a Gtk beginner. Are Application Resources really deprecated ?