Did you know WordPress creates thumbnails from the images you upload? You probably knew this — not everyone does, but most WordPress users are aware of this. š
Did you also know that when creating smaller images, they must be sharpened?
This is common knowledge for photography and website design professionals, but probably not for the majority of WordPress users.
And did you know that WordPress does not sharpen images when creating thumbnails?
I bet that you didn’t know this… not many people do.
WordPress creates several different thumbnail sizes by default (see your WordPress Settings > Media page for details) and potentially several more, depending on your theme and plugin settings. It’s not uncommon, for example, for WordPress to create a half-dozen (or more) images from the original image you upload. And in all cases, WordPress does not sharpen or make any adjustments to the resized image it creates!
But don’t “image optimization” plugins already do this?
No, they don’t — image optimization plugins are designed to create smaller / faster loading images, not better looking ones. And if you share a post / page to social media,
Source: https://managewp.org/articles/17529/wordpress-sites-create-fuzzy-thumbnails-solution
Willie Chiu's Blog 