The last parameter of [`update_user_meta()`][1], the previous value, is an optional parameter. If it's set it checks whether the value in the database is indeed the one you fed `update_user_meta()`. If you set that paramteter by grabbing the value from the database, it is completely redundant. So first off, omit that.

That being said, this is what solves your problem with overwriting:

    if( $_FILES['picture']['error'] === UPLOAD_ERR_OK ) {
        $upload_overrides = array( 'test_form' => false ); // if you don’t pass 'test_form' => FALSE the upload will be rejected
        $r = wp_handle_upload( $_FILES['picture'], $upload_overrides );
        update_user_meta( $user_id, 'picture', $r );
    }

The terminology is a bit confusing, since `UPLOAD_ERR_OK` is a status message and not an error, but that's how to check whether an upload was successful. If you make that the condition for saving the meta value, you're good to go.

For further reference on the [`$_FILES`][2] superglobal's errors or statuses, see [*Error Messages Explained* from the PHP manual][3].

**EDIT: How to get the URL of the uploaded image**

This edit caters to the expanded question in the comment to this answer.

    $pic_data = get_user_meta( $curauth->ID, 'picture', true );
    $pic_url = $pic_data['url'];

will save the URL into a variable, which thereupon can be echoed wherever you want. Assuming that `$curauth` is the current user object. You could use the global WordPress variable `$current_user` instead, but if you have the object already, might as well go with that.


  [1]: http://codex.wordpress.org/Function_Reference/update_user_meta
  [2]: http://php.net/manual/en/reserved.variables.files.php
  [3]: http://www.php.net/manual/en/features.file-upload.errors.php