To change your source folder:
- Select the blue folder icon from the Gallery
- Select 'Change Folder Location'
- Select your new Source Folder

The following occurs when you change source folder:
- If an image with the same name appears in the new folder, it is mapped to what is currently in Narrative.
- If an image isn't in the new folder, and isn't in the layout, it is removed from Narrative.
- If an image isn't in the new folder, but is in the layout, it remains, and will appear as a broken link.
- If an image is in the new folder, but not in Narrative it is added.
Alternative option:
Alternatively, you can add more images to your source folder by dragging images directly from finder on your Mac. See this article.