Please fill out the following sections to the best of your ability, it will help us investigate bugs if we have this information at the outset. Screenshots are especially helpful, so please provide those if you can.
if you want someone to troubleshoot this, you should send log files to iNat staff. they’re the only ones who will have any real chance of seeing what is wrong here. to send log files, go to the About page in the app, and tap 3 times in the Version section. that will give you a Debug Settings screen where one of the options is to Send Debug Logs.
that said, usually whenever i encounter a problem in the app, i usually just log out and then log back in, and whatever problem i had will be gone. (but, again, if you want someone to troubleshoot this, then you should send the log files.)
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”.