There is not an option (from what I can tell) to add alt text to images in a Timeline content type. This is an issue for accessibility, usability, older browsers, and search results. Most commonly, this will be an issue for users that are listening to the page via screen reader rather than navigating by sight. When an image does not have alt text defined (either alt="" or alt="my useful description here"), a screen reader will read the intire src of the image.
It would be great if alt text was either programmatically added, or if there was a field where content creators could provide alt text for their images.
Research if alt text is supported in timelineJS
If not determine if it’s possible to add it somehow.