Thanks for your feedback.
The images that you upload in Cascade Builder are not directly visible through "My Content". Our goal with that behavior is to avoid cluttering My Content with potentially a lot of items for those images and avoid issues with sharing permissions that could differ between those items and the story thus creating "broken" story.
You can manage those images through the Media Picker in the ArcGIS tab select "This story". All the images in your story should appear there and you can delete them. To replace an image in your story, you have to click the gear icon in the bottom right corner, chose the Manage Tab and click "Change Media". You will have to do that for every instance of the same image.
Unfortunately we currently have a bug in the application and not all your images will show up. Only the first 5 may shows. We are looking to fix that one in the next update in September.
If that behavior is not what you are looking for, user with a subscription (not public account), can also upload images from "My Content", this will create a new item in "My Content", please see the following blog post Adding and using photos and images in ArcGIS Online | ArcGIS Blog
