RichTextLabels in Godot don't support loading images from the filesystem out of the box. This repo contains example code how to do this in GDScript.

First, a new resource loader is registered. Then, RichTextLabel is extended such that when it's new property bbcode_with_images is set, all images are loaded using the above loader and only then the original bbcode_text property is set. This way, the bbcode_text handler can find the loaded resources and displays them correctly.

To test this yourself, import the project into Godot and edit the bbcode_with_images property to make the [img]-tag point to some image in your filesystem.