https://techcrunch.com/2019/12/11/twitter-will-now-preserve-jpeg-quality-for-photo-uploads-on-web/

Twitter is changing the way it processes uploaded images, and the new way of doing things will be much-appreciated by any photographers sharing their work on the platform. Twitter engineer Nolan O’Brien shared that the platform will now preserve JPEG encoding when they’re uploaded via Twitter on the web, instead of transcoding them, which results in a degradation in quality that can be frustrating for photo pros and enthusiasts.

There are some limitations to keep in mind – Twitter will still be transcoding and compressing the thumbnails for the images, which is what you see in your Twitter feed. But once users click through, they will get the full, uncompressed (at least, not additionally compressed) image you originally uploaded, provided it’s a JPEG.