I edited include/config_default.inc.php and made the same change suggested by Geno Shishkov and it worked for me.  Specifically:

$conf['picture_url_style'] = 'id-file';

Not only did it work, but also, previously it had been set to "id" and I was concerned than all previous links would be broken.  But, they still work even after the change.  To repeat, links formed based on id work when accessed using id-file.  That is some nice backward compatibility!