Hi @Raino , thanks for submitting! The Word JavaScript API doesn't support image captions at this time. But, the Word JavaScript API does support image alt text. The InlinePicture object offers the altTextDescription and altTextTitle properties.
If you'd like to request support for image captions, we use the Microsoft 365 Developer Platform Ideas system to track feature requests. Please check the existing ideas, and submit yours as a new idea if it's not already on the list. If the community upvotes your idea, it's more likely to be added to the add-ins platform.
Cheers,
Alison