Hi Warren,
Because of the way that images uploaded directly to your ArcGIS Online Organization account are stored, using many of them on a single webpage is unadvisable. You could instead upload them to your specific Shortlist app (this option is found under the "Add" button), or you could upload them to Flickr. In regards to your Shortlist, I do see all the images loading eventually, but it takes a couple minutes, and even after they've loaded, the app is really sluggish.
If you would like a more technical explanation of the problem, read on:
When you upload an image straight to your Organization and not an individual app, its url ends with /data. This url isn't actually where the image lives. Rather, if you enter it into your browser address bar, it redirects to the actual image address. You can see this happening if you watch your address bar.
For example:
http://www.arcgis.com/sharing/rest/content/items/f8fb52ac3e7e4c96a832a3161b7ffa29/data
redirects to:
https://ago-item-storage.s3-external-1.amazonaws.com/f8fb52ac3e7e4c96a832a3161b7ffa29/image_paysdaix...
This second link, ending in .jpg, is where the image actually lives. If you try to go to it directly from the link above, you'll get an "Access Denied" message. If you go through the /data url above, a one-time access token gets attached to the .jpg url, allowing you to see the image.
One or two of these redirected images on a page works OK, but trying to load more than about three of them at once slows down the page load enormously, and sometimes crashes the page (or the browser) entirely. As with most Story Maps, your Shortlist has 50+ images including thumbnails, so loading them all on a single page will be really problematic.
Here's a sample outside of Story Maps with just 10 of these redirecting images on the page. Notice how slowly they load. Try selecting the text below the images and notice how sluggish that interaction is. Sometimes the page will freeze for a bit, and sometimes you can even hear your computer's fan kick into overdrive while the page loads -- another sign the page is having troubles.
(If that link above doesn't work, try this: https://jsbin.com/qozikon/edit?html,output )
In conclusion, there are other ways to include your photos in your Shortlist app than uploading them to your Organizational account. Consider uploading them within the app or hosting them on Flickr.
(edit: The sluggishness of redirected images is greatly increased by the file size of the images. The app above had images that were very large. While we do not recommend using images uploaded directly to your ArcGIS Online Organization account, if you must do that we'd suggest resizing them so their file sizes are all under 100kb).
