php - Wordpress Function to change existing featured image size -

i using custom-community (cc) theme have created sub-theme have created functions.php file

the original cc theme has code in functions.php

        // theme uses post thumbnails     if (function_exists('add_theme_support')) {         add_theme_support('post-thumbnails');         set_post_thumbnail_size(222, 160, true);         add_image_size('slider-top-large', 1006, 250, true);         add_image_size('slider-large', 990, 250, true);         add_image_size('slider-responsile', 925, 250, true);         add_image_size('slider-middle', 756, 250, true);         add_image_size('slider-thumbnail', 80, 50, true);         add_image_size('post-thumbnails', 222, 160, true);         add_image_size('single-post-thumbnail', 598, 372, true);     } 

i want change post thumbnail size 222x160 larger size, e.g. 400x300.

i have read codex on post thumbnails, function reference/add image size, function reference/set post thumbnail size, function reference/the post thumbnail, plugin api/filter reference.

unfortunately understanding of php still limited , in early-learning phase.

i have seen examples of sound similar problems elsewhere twenty-ten theme, nothing can understand or apply myself without functions.php locking me out, deleting on ftp , starting again.

i still unsure if possible add function sub-theme functions.php either unset/re-assign image size thumbnail (or other specified image type) in parent theme.

to clarify functions.php , sub-theme working correctly can confirm have used functions styling login, lack understanding achieve myself.

following tuts-plus tutorial (point 6 - remove additional image sizes) tried following code in functions.php:

//change thumbnail size function remove_parent_image_sizes( $sizes ) {     unset( $sizes['post-thumbnail-size'] );     return $sizes; }  if ( function_exists( 'add_image_size' ) ) {     // 400 pixels wide , 300 px tall, cropped     add_image_size( 'post-thumbnails', 400, 300, true ); } 

i removed featured image post , applied new 1 still has dimensions 222x160.

still stumped :(

also tried way still no luck;

//change thumbnail size function remove_parent_image_sizes( $sizes ) {     unset( $sizes['post-thumbnail-size'] );     return $sizes; }  if (function_exists('add_theme_support')) {     add_theme_support('post-thumbnails');     set_post_thumbnail_size(400, 300, true);     add_image_size('post-thumbnails', 400, 300, true); } 

and adding line (modified stackexchange link) end caused site break:

add_action( 'after_setup_theme', 'remove_parent_image_sizes' ); 

you should first use remove_action() or remove_filter() functions remove parent theme behaviour , can add action of sub theme using add_action() . here nice tutorial on how modify parent theme behavior within child theme reference.