Image quality on Mastodon is terrible and there’s no indication in web client or apps what happens to your images (are they rescaled?) or if it even works to attach them for higher resolutions. Similar issues for 4K video.
This hits me often with astrophotography, where Mastodon absolutely trashes image quality. Other social networks (yes even “that one”) have no such issues.
There’s a lengthy issue thread here: https://github.com/mastodon/mastodon/issues/20255
(No, switching to another app isn’t a solution.)