I have finally found a solution. By changing the values of pipes position as follow, I've managed to make them appear properly on the screen.
SKSpriteNode* pipe1 = [SKSpriteNode spriteNodeWithTexture:_pipe1];
pipe1.position = CGPointMake( (-0.39*pipe1.size.width), 0 );
SKSpriteNode* pipe2 = [SKSpriteNode spriteNodeWithTexture:_pipe2];
pipe2.position = CGPointMake(self.frame.size.width *1.8, 0 );
The result ==> picture
The big challenge now is find a way to make the position of the pipes somehow random. :)