Avatar for users

Hy!

I’d like to add an avatar for my wordpress users.
The url to the avatar should be posted in the “usermeta” table with the meta_key “avatar”.

I’ve edited the users-edit.php

<fieldset>
<legend><?php _e('Avatar'); ?></legend>
<p><label><?php _e('Your avatar-url:') ?><br />
<input type="text" name="avatar" value="<?php echo $profileuser->avatar ?>" /></label></p>
</fieldset>

and now an input-field is posted in “users>>edit” (and if I’ve inserted an “avatar”-entry in the table the link is printed in the text-field for the avatar-url).

But I’m not able to update the entry in the sql-table. I’ve added update_usermeta( $user_id, 'avatar', $avatar); to the file “registration.php” but this has no effect.

Can you tell me which files or. functions I have to edit that the avatar-url is inserted into the table?