why i can't update user's picture by user_save() i wrote the program belowed

$fields=array('name'=&gt;'test user','picture'=&gt;$fid);


$account=user_save($account, $fields);

i really changed the user name but i can't change the picture field in the users table

what could the problem be?

by the way, i also use db_update. The situation is the same

the code is blowed

db_update('users')
-&gt;condition('uid',$account-&gt;uid)
-&gt;fields(array('picture'=&gt;$file-&gt;fid))
-&gt;execute();


still not work
I hope somebody can help me,Thanks!
Try this code 

 $file = file_load($fid);
 $file-&gt;status = 0;
 $edit = array ('picture' =&gt; $file);
 $u = user_save($account,$edit);