I just duplicated from https://www.inaturalist.org/observations/37123933 accessed via https://www.inaturalist.org/observations/identify?quality_grade=needs_id%2Ccasual&iconic_taxa=unknown%2CPlantae&place_id=65842 on Chrome 79.0.3945.86 running on ChromeOS.
403:
https://static.inaturalist.org/photos/58800768/original.jpg?1577778682
But if left-click on the image in the identification modal and then right click on that image to pull up the URL for the image, I get the image and the same URL:
https://static.inaturalist.org/photos/58800768/original.jpg?1577778682
I cannot see a difference in the URLs and when I refreshed the image on the left, I got a 403 error. But when I tried to replicate the “original image refresh leads to 403” I could not replicate. Thus I think this might be transient issue related possibly to server response, possibly connected to performance problems associated with large or complex taxon swaps?