Adding Controls to Form from another class

I have a program, which creates one pictureBox in Form1, and then creates an instance of a class that I called InitialState. The InitialState puts the source to the Image so that it is displayed, and after some time has passed, for which I used a Timer, it creates the next class, MainMenuState. Now, in that MainMenuState class that I've created, I would like to create another pictureBox and make it display on that Form1. Later on, I would like to make the pictures inside it change a bit, and then (possibly) destroy that pictureBox. After that, the program enters the next state (which is in yet another class), and again I would like that class to add a picture box to the original form, and so on.

Basically, I would like to dynamically add controls to the main Form1, but not in the said form, but from the classes I create later on. I've been searching on the internet for a way to do that, and it seems like I would have to use a delegate in order to invoke the Controls.Add method of the Form1 class. I've tried that, and the code compiles, but the pictureBox still doesn't show up.

Here's my code:

Form1 class:

 public const string RESOURCE_PATH = "C:/Users/Noel/Documents/Visual Studio 2010/Projects/A/Resources/Animations/";

public Form1()
        {
            InitializeComponent(); //here, the first pictureBox shows

            iInitializeComponent();
            zacetnaAnimacija.Dock = DockStyle.Fill; //zacetnaAnimacija is the first pictureBox that appears
            zacetnaAnimacija.Anchor = AnchorStyles.Top | AnchorStyles.Left;
            zacetnaAnimacija.SizeMode = PictureBoxSizeMode.StretchImage;
            InitialState intialState = new InitialState(this, zacetnaAnimacija); //entering InitialState
        }

InitialState class:

class InitialState : State
    {

        System.Timers.Timer initialTimer;
        PictureBox pictureBox1;
        Form1 form;

        public InitialState (Form1 form, PictureBox pictureBox1) {
            this.form = form;
            GifImage zacetnaSlika = new GifImage(Form1.RESOURCE_PATH + "Presenting.gif"); //this is just a .gif picture I'm displaying

            Image trenutnaSlika = zacetnaSlika.GetFrame(0); //a method that plays the .gif
            pictureBox1.Image = trenutnaSlika; //makes the first .gif display
            this.pictureBox1 = pictureBox1;            

            initialTimer = new System.Timers.Timer(2500);
            initialTimer.Enabled = true;
            initialTimer.Elapsed += new ElapsedEventHandler(OnTimedEvent);
        }

        private void OnTimedEvent(object source, ElapsedEventArgs e)
        {
            initialTimer.Enabled = false;
            MainMenuState menuState = new MainMenuState(form, pictureBox1); //enters main menu state with the Form1 argument passed on

        }

MainMenuState class:

class MainMenuState : State
    {
        Form1 form;

        public MainMenuState (Form1 form, PictureBox pictureBox1) {
            this.form = form;
            GifImage zacetnaSlika = new GifImage(Form1.RESOURCE_PATH + "animated.gif");

            Image trenutnaSlika = zacetnaSlika.GetFrame(0);
            pictureBox1.Image = trenutnaSlika; //this simply makes another .gif appear in the picture box instead of the first one

            PictureBox a = new PictureBox(); //HERE'S my problem, when I want to add ANOTHER pictureBox to that form.
            a.BackgroundImage = trenutnaSlika;
            a.Location = new System.Drawing.Point(0, 0);
            a.Name = "zacetnaAnimacija";
            a.Size = new System.Drawing.Size(150, 150);
            a.TabIndex = 1;
            a.TabStop = false;

            AddControl(a); //calling the delegate
        }


        public delegate void AddControls(PictureBox a);
        public void AddControl(PictureBox a)
        {
            if (form.InvokeRequired)
            {
                AddControls del = new AddControls(AddControl);
                form.Invoke(del, new object[] { a });
            }
            else
            {
                form.Controls.Add(a);
            }
        }

As I've said, the code compiles, but it doesn't create the PictureBox a on the Form1, when the MainMenuState is created. The thing is, if I don't use the delegate in the MainMenuState and just try to do something like form.Controls.Add(a), then I get a "cross-thread operation not valid" exception, and it doesn't even compile. That's why I used the delegate, but even now, it doesn't work.

Can someone please help me?