WordPress generate images size after upload

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I see 2 problems in your last part.

  1. You didn’t add hook for the code. Without it, I doubt that the add_image_size function will be executed. I suggest you wrap it in another function and add to after_setup_theme hook with add_action function.
  2. I think the way you use function_exists is unnecessary. With the way you set that conditional block, it will always true because add_theme_support is a built-in core function.

Here is my suggestion code.

if( !function_exists('theme_setup') ) {
    function theme_setup() {
        add_theme_support('post-thumbnails');
        add_image_size('test', 260, 380, true);
    }
    add_action('after_setup_theme', 'theme_setup');
}

It’s actually quite common way to interact your theme/plugin with WordPress core.