I have been experiencing this issue for a couple months now as well. Just updated the iNaturalist app, but still no luck. I also tried clearing the cache without effect. My internet connection is quite solid, so I do not think this is a possibility.
All photos are displayed normally when viewed in a browser. In the app, either/both of the thumbnail (in “My Observations” list) & images themselves (when viewing the observation) are affected. It seems that older photos are preferentially affected, but it is not so simple as “all photos before a certain date”.
I am on Samsung Galaxy A20 with Android 10.