For me, it seems random regardless of Opera or Firefox browser (but posting seems to work OK if I post images less than 1MB - just a guess as I haven't really tested the limits). I usually do direct image uploads to Lemmy rather than external URL links.
Update: 1mb file size is the limit that determines w!Heather you get the error or not.
It seems to work in Chromium, and when it fucks up in chromium you can delete the data of the app (cache) or reset to default and then it seems to work again.