functions.php

<?php

/**
 * Remove a imagem da tag <p> e adiciona em um nova tag com as classes has-image e no-padding. Adiciona a tag img a class img-responsive.
 * Pode ser prevenida através da adição de um atributo ou espaço em branco para <p>, por exemplo, <p class="lorem">ou mesmo <p >
 */
function updateTagPInContent( $content ) {
	return preg_replace_callback( '/<p>((?:.(?!p>))*?)(<a[^>]*>)?\s*(<img[^>]+>)(<\/a>)?(.*?)<\/p>/is',
		function ( $matches ) {
			/*
			Groups 	Regex 			Description
				<p>			starting <p> tag
			1	((?:.(?!p>))*?)		match 0 or more of anything not followed by p>
					.(?!p>) 		anything that's not followed by p>
				  ?: 			non-capturing group.
						*?		match the ". modified by p> condition" expression non-greedily
			2	(<a[^>]*>)?		starting <a> tag (optional)
				\s*			white space (optional)
			3	(<img[^>]+>)		<img> tag
				\s*			white space (optional)
			4	(<\/a>)? 		ending </a> tag (optional)
			5	(.*?)<\/p>		everything up to the final </p>
				i modifier 		case insensitive
				s modifier		allows . to match multiple lines (important for 1st and 5th group)
			*/

			// image and (optional) link: <a ...><img ...></a>
			$image = $matches[2] 
			         . preg_replace( "/<img(.*?)class=\"(.*?)\"(.*?)>/is", 
									 '<img$1class="$2 img-responsive"$3>', 
									  $matches[3] ) 
			         . $matches[4];
			// content before and after image. wrap in <p> unless it's empty
			$content = trim( $matches[1] . $matches[5] );
			if ( $content ) {
				$content = '<p>' . $content . '</p>';
			}

			return '<p class="has-image no-padding">' . $image . '</p>' . $content;
		}, $content );
}

add_filter( 'the_content', 'updateTagPInContent' );