Rewrite photo field URLs
theme_image() by default looks like this:

function theme_image($variables) {
  $attributes = $variables['attributes'];
  $attributes['src'] = file_create_url($variables['path']);

  foreach (array('width', 'height', 'alt', 'title') as $key) {

    if (isset($variables[$key])) {
      $attributes[$key] = $variables[$key];
    }
  }

  return '&lt;img' . drupal_attributes($attributes) . ' /&gt;';
}


so, if you are storing the images in this field in an identifiable place, eg, you can look at $variables['path'] and figure out they are the ones you want to always look for on a specific host, you should be able to do something like this in your theme:

function MOTOTHEME_image($variables) {
  $attributes = $variables['attributes'];

  if (dirname($variables['path'])==WHEREIKEEPTHISFIELDSIMAGES) { // on d7 this might be a uri so you might need to get fancier than dirname() :)
    $attributes['src'] = // http://.....
  } else {
    $attributes['src'] = file_create_url($variables['path']);
  }

  foreach (array('width', 'height', 'alt', 'title') as $key) {

    if (isset($variables[$key])) {
      $attributes[$key] = $variables[$key];
    }
  }

  return '&lt;img' . drupal_attributes($attributes) . ' /&gt;';
}