As a GTK beginner, I didn’t notice that resource files were deprecated when I wrote the tutorials. 
Looking at the documentation for gtk_image_new_from_resource, it says that it was introduced in GTK 3.4, so it seems unlikely that resource files were deprecated in 3.0. Where did you see the warning?
The GTK documentation suggests using GtkCssProvider instead, which I tried to cover in an example. That example uses GResource.