The National Park Service has a public online gallery where staff can upload and share photographs and other items. I've uploaded many photos and pdfs there and used them successfully in the Journal, Series, and Tour story maps and the Web AppBuilder. The photos always load quickly and the popouts are high resolution.
But when I paste a URL from the gallery into Cascade, the builder insists that isn't a photo. Here's an example that is in use in a Map Journal app, but Cascade rejects it. https://focus.nps.gov/GetAsset/a6849ecb-2962-4a87-b090-931254c679b9. Yes, I realize that the URLs don't end with jpg or other format, but that has never been a problem in other apps.
Any suggestions welcome.