Use WHERE in an INSERT INTO statement

Use WHERE in an INSERT INTO statement

I'm trying to let users insert a subject to an image. I know I can't use a
WHERE clause in combination with an INSERT INTO statement, and I know that
I should use SELECT. I am very new to mysql and I didn't understand the
results I tried to search on google. So I needed a more specific answer;)
$classed = mysql_query("INSERT INTO images (subject) VALUES
('$_POST[subject]') WHERE image_id='$_POST[id]'");