It happens if I try to upload an unedited photo from my camera roll. If I crop it, even by a tiny amount, it lets me upload. It's a really frustrating issue because I have to edit every photo before uploading.
OP, check if the default format of your pictures is HEIF (Samsung I know does this). It can be found under the camera app settings typically.
Mastodon and some other servers choke on this format frequently unless all the backend code is updated... Not sure about the Lemmy servers yet. It drove me nuts for days until a log item in my mastodon server I run brought my attention to it.
Unfortunately if you confirm this, you can't fix it client side except to convert your pictures first or disable that format for saving.
