Hi Gumbanomicon,
it seems that you wrote Hexagram.png when defining the name of the background image. You just need to write Hexagram. The plugin then automatically loads the .png file.
Just check out the tree with the tree editor or search in the Techtrees.json for "bgimg". You should find something like "bgimg": "Hexagram.png". Just remove the .png and it should work.
Best regards,
Felski