MySQL foreign key constraint

Suppose we have users table:

CREATE TABLE users
(
user_id int NOT NULL,
user_name varchar(45) NOT NULL,
PRIMARY KEY (user_id)
)

We have another table  pictures,  user_id is foreign key which is primary key in table users.
Using foreign key to prevent  insert user_id in table pictures which is invalid in table users
CREATE TABLE pictures
( 
pic_id  int NOT NULL,
user_id int NOT NULL,
PRIMARY KEY (pic_id),
 FOREIGN KEY (user_id) REFERENCES users(user_id)
)

We can use  naming of foreign key constraint, which can be used in drop foreign key
CREATE TABLE pictures
( 
pic_id  int NOT NULL,
user_id int NOT NULL,
PRIMARY KEY (pic_id),
CONSTRAINT fk_pictures FOREIGN KEY (user_id) REFERENCES users(user_id)
)

Now we can drop the FOREIGN KEY constraint using the following SQL:
ALTER TABLE pictures DROP FOREIGN KEY fk_pictures

When the primary key is updated or deleted on parent table, what is the behavior of the child table?
 We have four options:

  RESTRICT | CASCADE | SET NULL | NO ACTION

CASCADE: automatically delete or update the child table when the parent table is deleted or rejected
SET NULL: the foreign key table is set NULL  child table when the parent table is deleted or rejected
RESTRICT:  Rejects the delete or update operation for the parent table
NO ACTION: the same as RESTRICT
EXAMPLE:
CREATE TABLE pictures
( 
pic_id  int NOT NULL,
user_id int NOT NULL,
PRIMARY KEY (pic_id),
CONSTRAINT fk_pictures FOREIGN KEY (user_id) REFERENCES users(user_id)
ON DELETE CASCADE ON UPDATE CASCADE
)

 INSERT INTO `users`  VALUES
('1', 'jiansen'), ('2', 'andy')
INSERT INTO `pictures`  VALUES('1', '1')
will be OK
INSERT INTO `pictures`  VALUES('2', '3')
will fail due to that   user_id=3 is not in users table.
Reference:
http://www.w3schools.com/sql/sql_foreignkey.asp
http://dev.mysql.com/doc/refman/5.5/en/create-table-foreign-keys.html