css - How to Lower the Quality of Wordpress Featured Image (PHP) -

all posts find people complaining image quality low images high quality , lower them increase page speed. how set resolution 300 width auto height in functions.php file? here code using call in functions.php:

(note): not want hard crop or soft crop, want affect resolution.

php:

<?php  function ericshio_theme_setup() {     add_theme_support( 'menus' );     add_theme_support( 'custom-logo' );     add_theme_support( 'custom-header' );     add_theme_support( 'custom-background' );    /* post thumbnails */      add_theme_support( 'post-thumbnails' );     add_theme_support( 'post-formats', array( 'aside', 'image', 'video' ));     register_nav_menu('primary','main navigation'); }  add_action('init', 'ericshio_theme_setup');  ?>