Letting the Plugin generate and send the covers is a good solution. It uses a much more sane Jpeg compression level, (approx. 80)
Here is comparing the image files from a single book with different sending options:
Edit: There is a problem with this board and PNG images,, they were scaled to twice their size. You can get the screenshots here:
https://transfer.sh/LXQbN/screenshots.7z
(I removed the images before the mods did.)