Yesterday we have talked about 2 normal ways to render the user profile picture in Drupal 7.
Drupal 7 – Render user profile picture programmatically @ 1

Unfortunately you can only get the user profile picture with default size if you have enabled the Gravatar integration moddule. i.e

$user = user_load($uid);
// This works for user with Gravatar linked
print theme('user_picture', array('account' => $user));

// This DOES NOT work for user with Gravatar linked
print theme('image_style', array('path' => $user->picture->uri, 'style_name' => '<image-style>'));

Seems we need something abnormal. O.o

So i spent some time on Google. Finally i could solve the problem by using the Imagecache External module.

After you have enabled the module, go to the permission settings and enable the View external images as well as Fetch external images for eligible roles. Then you can render the Gravatar picture by

$user = user_load($uid);
print theme('imagecache_external', array('path' => gravatar_get_gravatar($user ->mail), 'style_name'=> '<image-style>')); 

The following is a Views example which i want to show a list of users which some of them are linked to Gravatar and some don’t.
1. Add a User: Uid field and exclude it from display

2. Add a Global: PHP field (require Views PHP module)

3. Set the following code in the Global: PHP field

<?php
  $user = user_load($row->uid);
  if (empty($activity_user->picture)) {
    print theme('imagecache_external', array('path' => gravatar_get_gravatar($user ->mail), 'style_name'=> '<image-style>')); 
  } else {
    print theme('image_style', array('path' => $user ->picture->uri, 'style_name' => '<image-style>'));
  }
?>

Done =)

Reference:

• Render user picture programmatically with image style
• Display Gravatar Code Documentation
• DrupalContrib – gravatar_get_gravatar
• Drupal module – Gravatar integration
• Drupal module – Imagecache External
• Drupal module – Views PHP