How to override the default image style for displaying user picture?
Use hook_preprocess_user_picture(). See template_preprocess_user_picture().

Here a sample code:

/**
 * Implements hook_preprocess_user_picture().
 *
 * @see template_preprocess_user_picture().
 */
function example_preprocess_user_picture(&amp;$variables) {
  $style = 'your_style';

  $account = $variables['account'];
  if (is_numeric($account-&gt;picture)) {
    $account-&gt;picture = file_load($account-&gt;picture);
  }
  if (!empty($account-&gt;picture-&gt;uri)) {
    $filepath = $account-&gt;picture-&gt;uri;
  }

  if (isset($filepath)) {
    $alt = t("@user's picture", array('@user' =&gt; format_username($account)));
    $variables['user_picture'] = theme('image_style', array('style_name' =&gt; $style, 'path' =&gt; $filepath, 'alt' =&gt; $alt, 'title' =&gt; $alt));
  }
}