change 350x250 thumbnail rendered size

add_filter( 'post_thumbnail_size', 'change_jnews_thumbnail', 99, 2 );


function change_jnews_thumbnail( $size, $post_id ) {
    if ( 'jnews-350x250' === $size ) {
        $size = 'jnews-750x536'; /* you also can change this with jnews-1140x815 */
    }
    return $size;
}