Can´t get the author´s picture printed onto a node in Drupal 7

After reading the documentation, it seems that the correct way of doing that is like this:

<?php print $user_picture; ?>
Wich won´t work (it doesn´t print anything at all).

Afetr asking around, I get this answer, wich sadly won´t work either:

    <?php global $user;
    $image = theme('user_picture', $user);
    print $image;?>

That code was suggested as a "last resource", but when I try to use it, I get this error message:

    Fatal error: Cannot use object of type stdClass as array in /includes/theme.inc on line 1054

Looking for that particular line: if `(isset($variables['#theme']) || isset($variables['#theme_wrappers'])) {`, and in context:

      if (isset($variables['#theme']) || isset($variables['#theme_wrappers'])) {
        $element = $variables;
        $variables = array();
        if (isset($info['variables'])) {
          foreach (array_keys($info['variables']) as $name) {
            if (isset($element["#$name"])) {
              $variables[$name] = $element["#$name"];
            }
          }
        }

It´s pretty clear that it should work, but it doesn´t.

Using contemplate module, I get this output as an alternative:

<?php print $node->picture ?>

Wich won´t work either.

I´ve tried then with Devel module (using the /devel tab of the node):

I get this:

    picture (String, 4 characters ) 2876
    $...->picture

Wich won´t work, because the output is just "2876", but is the only "picture" there.

So what can I do? How can I trace this problem?

Thanks for your help!