How introduced Form and its components into Class?

You first of all need to get a reference to the Form object. There are many ways of doing this but here's one which doesn't require existing code to be altered:

  Form1 f1 = (Form1)Application.OpenForms["Form1"];

The controls on the form, which are normally private, then need to be made accessible to code in the class. An easy way to do this is to change their Modifiers property (in the Properties Box) from Private to Internal.

You can then access your PictureBox as follows:

  PictureBox pb = f1.pictureBox1;

Finally, make sure that you have this 'using' directive at the top of your class file:

  using System.Windows.Forms;