The WordPress function add_image_size allows the declaration of a size name to specific dimensions (see WordPress reference). Generally, these are declared in the functions.php file. But what if there is a declaration in the parent theme that needs to be changed in the child theme? Unfortunately, in this case, it will not be overwritten. To do so, add_image_size needs to be defined in a function on both the parent and child.
In the parent functions.php:
function parent_image_size () {
add_image_size( ‘name-one’, 600, 300, true );
}
add_action( ‘after_setup_theme’, ‘parent_image_size’ ,10);
In the child functions.php:
function child_image_size() {
add_image_size( ‘name-one’, 300, 200, true );
}
add_action( ‘after_setup_theme’, ‘child_image_size’, 11 );
This scenario works by using the add_action priority attribute. The child function will execute after the parent function, thus using the child size(s).
If existing images need to be resized, you will need to run a regenerate images plugin.
Plugin Solution
In lieu of the code above, a comprehensive plugin that works well is Simple Image Sizes. This plugin will display a table of all defined image sizes and allow change the sizes. In addition, it feature a image regeneration with lots of options for where to apply the sizing change.