Maybe there was some special badge or indicator that showed up before but has now disappeared since Instagram got rid of the feature.

I don't use Instagram either but a Google image search shows a "digital collectible" checkmark in the bottom left corner. I hope this wasn't the whole thing as I don't see what would stop you from putting this checkmark into the image.

Was also wondering the same thing, but it seems to only show up in-app. If you're not a user and just viewing on the web I don't think you see anything.

And yes it says "Digital Collectible" and the image animates back and forth in a partial spin before settling into place.