That's great that USFS is focusing on accessibility! Many government agencies at all levels, as well as commercial and non-profit organizations, are working to meet accessibility standards, and we agree it is very important.
Esri has has done a lot of work to make ArcGIS products both accessible themselves and capable of creating content that meets accessibility standards and guidelines. Here are some resources for learning more about accessibility and ArcGIS StoryMaps that might be helpful as you do more work in this area.
In the case of the collection thumbnails, I think it's important to note that accessibility guidelines require an alt attribute for every image. However, images that serve a decorative purpose do not need author-provided alt text, which is why it isn't currently in our plans to add a way for authors to enter alt text for collection thumbnails. (One reference is here, but you can find this in many other places, including 508 guidelines here.)
The intended purpose of collection thumbnails is to make the overview page more engaging for sighted readers. But since each item has a title, which is read by screen readers, the images are considered decorative because the title conveys all the information needed for non-sighted readers to distinguish each item. If non-sighted readers also had to also listen to image alt text for each item on the overview page we feel this would make their experience needlessly complex.
From a technical perspective, as mentioned above, accessibility guidelines say that decorative images should have an empty alt tag in the HTML. This is something that ArcGIS StoryMaps does automatically for you. For example, you can see the empty alt tag on the collection item thumbnail in the screenshot, below.
I believe this should address your concern about the requirement for all images to have alt text, but let me know if you have additional questions.
Owen Evans
Lead Product Engineer | StoryMaps