Low quality thumbnails

With the standard 300 pixel width images is all right, but whe nI go to resize it larger to 555 pixels, then the image is looking to be at a very low qualiy, when instead clickc on it, the e zoom is ok: http://spesagodina.it/web/i-nostri-reparti/la-cantina/
Please help!
Also I created a new thumbnail to show on posts, but I don’t find it in the select menu of the edit pages..

https://ww.wp.xz.cn/plugins/simple-image-sizes/