Thanks. This makes things… sort of more clear but not by much. Bc I’ve joined an atheism community which apparently is on .world and I can comment there just fine. But at the same that post I mentioned opens at .world and says I must login. (scratching my head where it doesn’t itch 😂) Still, thanks to all three of you for the answers, I wasn’t sure if my question will be answered or deleted.

Yep, that was it. Apparently 1.1 MB PNG was too big. Thanks!

Found it. But it won’t accept the image which is weird. Asklemmy has a cover image (or wallpaper) which is 1536x768 - the same dimensions I’m trying to upload and the site still rejects it because “it’s too large”.