Put the new code in a new fit_to_frame() method inside CameraMovement.gd. The old code did not work properly for images with width larger than their height.
Made a new "src" folder that will contain the source code files, like all the GDScript and scene files. Please read this for more details: https://www.gdquest.com/docs/guidelines/best-practices/godot-gdscript/
It made no sense to keep scenes separate from their scripts. More file organizing will follow soon.