Quality of featured images always decreases?

Apologies for consulting a topic that is very repeated. I have reviewed the following items but I have not solved my problem:

https://generatepress.com/forums/topic/number-and-quality-of-images-generated-by-gp/
https://docs.generatepress.com/article/adjusting-the-featured-images/
https://generatepress.com/forums/topic/losing-quality-in-wordpress/
https://generatepress.com/forums/topic/featured-image-6/

I uploaded the featured images in the exact size they appear in my blog (340x230) and lose a lot of quality: https://oficientes.com/blog/

Also, in the home the images are resized to 331x224 and I do not know why ... These entries I generate them with the plugin WP Show Posts: https://oficientes.com

I have also uploaded the outstanding images in large size (1200x811), with the same results: lose a lot of quality.

What I can do? I do not use any plugin to optimize images in Wordpress.

Example of loss of quality in all sections: https://i.imgur.com/7ggRcpE.jpg

Thank you very much for your patience!