Owen Evans on the Story Maps team writes...
Accessibility is an important goal at Esri, and in recent ArcGIS Online updates the Story Map Journal and Story Map Series app received significant enhancements related to accessibility.
Read on to learn more about these new features and how they work to help you create accessible stories.
Alt text can be added to a new story as you author it, or it can be added to stories you’ve already published. Just add a concise description of the media to the Alternative Text box when you are adding or editing it in the Map Journal builder.