Attach code to image with named attributes

You could hook the filter ‘the_content’. Your filter function will be passed the entire post string just before it is echoed out to the screen. With some clever string search and replace, you can achieve your goal.

Just to let you know that, not finding any other information, I followed your advice successfully.
I used this https://codex.www.ads-software.com/Plugin_API/Filter_Reference/the_content and this
https://php.net/manual/en/ref.strings.php .
Maybe I might as well paste the code I wrote here, in case somebody stumbles upon this post and gets curious about my smudgy bites:

function modify_capa($content) {
 	if(in_array('get_the_excerpt', $GLOBALS['wp_current_filter'])) return $content;
	if (strpos($content,'alt="capa"')): ?>
		<?php

			// FIND CAPA
			$capapos = stripos($content,'capa');
			$imgend = strpos($content,'>',$capapos);
			$imgend = $imgend;
			$content_length = strlen($content);
			$imgend_fromend = -1*($content_length - $imgend);
			$imgstart = strrpos($content,'<',$imgend_fromend);
			$img_length = $imgend - $imgstart+1;
			$capa_img_tag = substr($content, $imgstart, $img_length);

			//FIND ATTACHMENT ID
			$id_start = strpos($capa_img_tag,'wp-image');
			$id_end = strpos($capa_img_tag,' ',$id_start);
			$id_string = substr($capa_img_tag, $id_start,$id_end-$id_start);
			$capa_id = substr($id_string,strrpos($id_string, '-',-1)+1);

			$capa = wp_get_attachment_image_src( $capa_id,'publication');
			$new_capa = '<img class="capa-shadow" src="'.$capa[0].'" alt="capa" width="'.$capa[1].'" height="'.$capa[2].'" />';
			echo $new_capa;
		?>

		<?php //INSERT NEW CODE ?>
<div> HTML stuff <?php echo 'with php stuff'; ?> </div>

		<?php 

		//REMOVE OLD CAPA
		$content = str_ireplace($capa_img_tag,'',$content);

	endif;

	return $content;

}

add_filter( 'the_content', 'modify_capa' );