The help page for configuring pop-ups touches on this briefly, but it does state:
"Optionally enter the URL to an image stored on an external website, in ArcGIS Online, or as an attachment within the layer's data. PNG, JPEG, and GIF images are supported. If the image is stored as an item in ArcGIS Online, you must enter the URL copied from the URL field on the item page, which is only available if the image item is shared with the public."
So, unfortunately that still doesn't offer a solution when the images need to be only visible within the organization. I did uncheck the "refresh interval" box under the pop-up image configuration, but after I save and refresh the page, it appears to be checked again.
Best,
Katie
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