add_action( 'init', 'mindfulmum_setup' );
function mindfulmum_setup() {
    set_post_thumbnail_size( 66, 66, true ); // Normal post thumbnails
    add_image_size( 'single-post-featured-image', 618, 9999 ); // Permalink thumbnail size
    add_image_size( 'homepage-featured-image', 960, 240 ); // Homepage featured image
}