need to replace default drupal user picture with an image field from user profile
how to render the image???


Sounds like you're probably looking for image_style_url(). Assuming you have an image preset called thumbnail, try something like this:

$picture_style = 'thumbnail';
$picture_uri = $account-&gt;field_profile_picture['und'][0]['uri'];
$variables['user_picture'] = image_style_url($picture_style, $picture_uri);