How to replace timthumb with wordpress thumbnails for attachment-images in wordpress themes..

functions.php

// add the thumbnail-sizes you need in functions.php
if ( function_exists( 'add_image_size' ) ) {
  // repeat for all the thumbnail-sizes you need 
  add_image_size( 'thumb790x450', 790, 450, true );
}
// helper-function for getting the attachemnt-ID from source. You need this when you only have an image-src
function get_attachment_id_from_src ($image_src) {
  global $wpdb;
  $query = "SELECT ID FROM {$wpdb->posts} WHERE guid='$image_src'";
  $id = $wpdb->get_var($query);
  return $id;
}

theme-file.php

/* in any theme/plugin-file where timthumb is used */

// If you only have the url (here named $src), use get_attachment_id_from_src
$photoID = get_attachment_id_from_src($src);

// When you have the attachment-id (here named $photoID):
$thisthumb = wp_get_attachment_image_src( $photoID,"thumb790x450");
$thisthumb = $thisthumb[0];
echo '<img src="$thisthumb" />';