Where do these avatars come from?

In yuor comments.php, you should have something similar.. change URL to the url of the image you want to be default

<?php
   echo get_avatar( get_comment_author_email(), $size = '50', $default = 'URL' );
   ?>

I wanted to have gravatar or mybloglog avatars in that place, which i see on many other blogs, like this one – https://www.lifeiscolourful.com/june-earnings-addition-of-2000-in-money-meter/

Using that php snippet will display the commentor’s gravatar if they have one. If not, you can specific the default to show.

I got it.
Tried to play with the codes there and gravatar worked in..