Answered: How to save avatar images in original and good quality?

It doesn't make them low quality. It resizes them for specific sections of the website, by adding a query string to the image URL. So when you stretch an image in one of these sections to make them bigger, they may look blurry.

question2answer.org/qa/?qa=image&qa_blobid=1809957586498882777&qa_size=50

How to change this value?

You can change image size values of individual sections such as: user profile page, top users page, questions, answers, comments, question lists, message lists.

Go to Admin > Users > scroll down to "Avatar size on" section.

Edit

PS: After updating image sizes on that section and there's still one specific user that their image still appears blurry, it's most likely they've uploaded a very small image, like 30x30 pixels , and now it's getting stretched to a bigger size.