It's likely the site itself, as the .jpg extension is referenced in the html. The screenshots should be pngs, not jpgs. Weirdly the author is using a mix of bot

It's likely the site itself, as the .jpg extension is referenced in the html. The screenshots should be pngs, not jpgs. Weirdly the author is using a mix of both - it could be that their site automatically encodes larger images as jpgs. The full res jpg versions are heavy with artifacts.