I have embedded an ArcGIS Online configurable web mapping application into my StoryMap, so it suggests that I include an alternative image for when viewing on a mobile device. So I took a screen shot of the standalone web mapping app and uploaded it. It appears to upload without issue, so then I publish the StoryMap. Then, I go to preview in in mobile mode, and the uploaded image is not loading.
When I go back to edit mode to see what has happened, the uploaded alternative image is no longer there. What is going on? There is no mention on the editing interface nor in the StoryMap documentation about image sizes or formats. I am using a PNG image. Any ideas out there?