Wordpress Define Child Theme Thumbnail Dimensions

To define the thumbnail image dimensions for your Wordpress child theme add this function to your child theme's functions.php file

if ( function_exists( 'add_theme_support' ) ) {
    add_theme_support( 'post-thumbnails' );
    set_post_thumbnail_size( 150, 150, true ); // default Featured Image dimensions (cropped)

    // additional image sizes
    // delete the next line if you do not need additional image sizes
    add_image_size( 'themeName-medium', 400, 220 ); // 400 pixels wide 220 pixels high
 }