When publishing your Narrative Publish post via Narrative's publishing service, the HTML for your post is presented via Javascript to show the Narrative post on your website.
When Google scrapes your website for SEO data it reads all HTML and Javascript on your website including the Narrative HTML.
This means that all your SEO data including the image descriptions, body text and the images themselves are seen by Google.
When somebody searches within Google image search, your images will display as usual and the location of the image will be represented as your website.
For more information, check out our SEO Guide!