Filter the_content() to add Pinterest button code, by Jan Dembowski.

pinterest-img-tag-wordpress-filter.php

<?php
/*
Plugin Name: Pinterest Img Tag Button
Description: This will wrap images in the_content with Pinterest Button code
Author: Jan Dembowski
Author URI: http://blog.dembowski.net/
Version: 0.5

From http://wordpress.org/support/topic/add-code-before-each-image

*/

add_filter( 'the_content' , 'mh_wrap_image' , 15 );

function mh_wrap_image( $content ) {

// Regex to find all <img ... > tags
$mh_img_regex1 = "/\<img [^>]*src=\"([^\"]+)\"[^>]*>/";

// Regex to find all <a href"..."><img ... ></a> tags
$mh_img_regex2 = "/<a href=.*><img [^>]*src=\"([^\"]+)\"[^>]*><\/a>/"; 

// Populate the results into 2 arrays
preg_match_all( $mh_img_regex1 , $content, $mh_img );
preg_match_all( $mh_img_regex2 , $content, $mh_matches );

// The second array will be a subset of the first so go through 
// each element and delete the duplicates in the first.
$i=0;
foreach ( $mh_img[0] as $mh_img_count ) {
  $i2=0;
  foreach ( $mh_matches[0] as $mh_matches_count ) {
    if ( strpos($mh_matches_count, $mh_img_count ) ){
      unset( $mh_img[0][$i] );
      unset( $mh_img[1][$i] );
            $i2++;
      break;
    }
          $i2++;
          }
  $i++;
  }
// There is almost certainly a better way to do this.
// Append the no links array to the $mh_matches array.
$i=0;
$mh_start = count( $mh_matches[0] );
foreach ( $mh_img[0] as $mh_img_count ) {
  $mh_matches[0][ $mh_start + $i ] = $mh_img_count;
  $i++;
}
$i=0;
foreach ( $mh_img[1] as $mh_img_count ) {
  $mh_matches[1][ $mh_start + $i ] = $mh_img_count;
  $i++;
}

// If we get any hits then put the code before and after the img tags
if ( $mh_matches ) {;
  for ( $mh_count = 0; $mh_count < count( $mh_matches[0] ); $mh_count++ )
    {
    // Old img tag
    $mh_old = $mh_matches[0][$mh_count];

    // Get the img URL, it's needed for the button code
    $mh_img_url = $mh_matches[1][$mh_count];

    // Put together the pinterest code to place before the img tag
    $mh_pinterest_code = '<span class="pinterest-button"><a href="http://pinterest.com/';
    $mh_pinterest_code .= 'pin/create/button/?url=' . urlencode( get_permalink() );
    $mh_pinterest_code .= '&media=' . $mh_img_url . '&description=';
    $mh_pinterest_code .= urlencode( get_the_title() ) . '" class="pin-it-post"></a>';

    // Replace before the img tag in the new string
    $mh_new = preg_replace( '/^/' , $mh_pinterest_code , $mh_old );
    // After the img tag
    $mh_new = preg_replace( '/$/' , '</span>' , $mh_new );

    // make the substitution
    $content = str_replace( $mh_old, $mh_new , $content );
    }
  }
return $content;
}