@katherine_evans -- We looked into this and most of these images are compressed JPGs and don't look clear to me even in their original form.
The two PNGs you provided are the only images that are sufficient quality and format to use for images on the web with text. However, they are much larger than needed (3300px wide), so to ensure the images load quickly when readers view your story the builder has to resize them. This sometimes can have a slight effect on their appearance. In our testing, we do notice a slight difference between the original and how the image looks in a story, but the text is still clear. I'd recommend exporting the images at a size closer to 2000px wide, which will give you a result that should be closer to what you'd expect to see in the builder and provide clear text in the images.
Please also see this article which has some good information and tips on working with and preparing images for use in a story. A Guide to Working with Images in ArcGIS StoryMaps (esri.com)
Owen Evans
Lead Product Engineer | StoryMaps