ITEM DISPLAY IMAGE and ITEM DISPLAY THUMBNAIL Issue

Currently in SC, I use the following format to name image files displayed as an item's Thumbnail image and Product Detail Page (PDP) image(s) in the webstore:

[Item Name/Number]_{Category 1}_{Category 2}.[EXT]

No problem here.

However, I would like to use one image for multiple items. So created such image and uploaded it to both the Images and Web Images folders via File Cabinet's Add File Image button. I then opened the item record and manually assigned the image file to both the ITEM DISPLAY IMAGE and ITEM DISPLAY THUMBNAIL fields and saved the record.

Subsequently however, I continue to get "Image Not Available" for both the item's Thumbnail image and PDP image. Perhaps my concept on how ITEM DISPLAY IMAGE and ITEM DISPLAY THUMBNAIL images are to be used is wrong?