@thomasfuchs
I've been resizing photos to the maximum pixel dimensions Mastodon accepts prior to posting, so the server isn't doing its own, poor quality resampling.
Obviously doesn't fix the underlying issues, but it avoids the horrendous, additional smearing of high-frequency detail.
Basic notes: 2.0 megapixels maximum, JPEG with highest possible quality - Mastodon will still recompress the image, but *not* resample it.
(4K OLED display here, so yeah - I see the problems...)