I finished implementing the background image in GraphWidget. I attached the files so you can see the result. You just need to run the file named test_window.py within an environment with graph-tool installed. I think that it only works with PNG image files, because I used the method
During this development I have found some difficult to use the key press events when the GraphWidget is inside a Gtk container. I was trying to set a GraphWidget inside the Gtk.Overlay, which is set inside a Gtk.Notebook page. In this configuration all the events with scroll and control key worked perfectly, but when I press the keys 'a', 's' or 'r' nothing happens. Do you have any idea why this occurs?
I had another problem related with the docker image when I was developing the GUI. All the text outside the GraphWidget was shown as a undefined character, like small squares. I thought that happened because the Arch Linux is messing with the fonts. So I created a new Dockerfile using Ubuntu 16.04 and compiled with graph-tool. This way I had no problem with the font.