add_filter( 'post_thumbnail_html', 'remove_width_attribute', 10 );
add_filter( 'image_send_to_editor', 'remove_width_attribute', 10 );

function remove_width_attribute( $  html ) {
   $  html = preg_replace( '/(width|height)="\d*"\s/', "", $  html );
   return $  html;
}

Thanks to Damien for the cool hack!

<?php
   if (is_plugin_active('plugin-directory/plugin-file.php')) {
      // the plugin is active
   }
?>

Thanks to Cats Who Code for the cool tip!