The inventory system currently uses load() for item icons. Thus only images that use Godot's resource system can be used for items. Don't know how well this works, but someone made a tutorial for adding such images: https://f95zone.to/threads/strive-for-power-v0-5-25-maverik.1136/page-100#post-1...
Otherwise, your other option would be to change load() for image icons to globals.loadimage(), thus enabling the usage of normal image files for items.