Hi Trevor -- Thanks for sharing your images. Your assessment is correct; those images are not large enough to use as background images in a Cascade. If you inspect them you can see the largest one is only 640 pixels wide (and one is just 240 pixels wide). To use an image as a cover or immersive background image it should be about 2000 pixels wide.
You may be able to use those images in your story if you add them in-line to a narrative section and set the size to SMALL.
What model iPhone are you using to take photos? If you are using an iPhone 3G or later the photos should be at least 2000 pixels wide. Sometimes emailing photos will reduce their quality. If so, be sure to send the full quality images when you transfer them to the computer with which you are making your story map.
More information about recommended image sizes for all story map apps can be found in our FAQ. For example, here's information about image sizes/treatments in Cascade.
Owen Evans
Lead Product Engineer | StoryMaps