Default thumbnail doesn’t appear

@vegroland I don’t see a problem with the behavior. Next time you come across this, can you please save the page as an HTML and share it here?

@vegroland this is happening because the feed contains an image as shown here but that image points to an XML. So the plugin is behaving correctly. If you want to override this behavior and instead force show the default image, you can use this

		add_filter('feedzy_thumb_output', 'feedzy_thumb_output', 10, 4 );
		function feedzy_thumb_output($contentThumb, $feedURL, $sizes, $item){
			if ( strpos( $contentThumb, 'http://www.elmundo.es/rss/index.xml' ) !== false ) {
				$contentThumb = 'https://buscarnoticias.es/wp-content/uploads/2020/03/buscarnoticias_favicon.jpg';
			}
			return $contentThumb;
		}

@vegroland try this:

		function feedzy_thumb_output($contentThumb, $feedURL, $sizes, $item){
			if ( strpos( $contentThumb, 'http://www.elmundo.es/rss/index.xml' ) !== false ) {
				$contentThumb = '<img src="https://buscarnoticias.es/wp-content/uploads/2020/03/buscarnoticias_favicon.jpg">';
			}
			return $contentThumb;
		}