How to change location of form2 from form1

Depends on how you create the two forms.
Since you have called them "form1" and "form2" the chances are that you created the form2 instance in form1 code, and used form2.Show to display it.
In which case it's pretty simple: use the instance of form2 that you created, by stroring it in a class level variable in form1:

private Form2 form2 = null;
...
   form2 = new Form2();
   form2.Show();
...

Then, in your PictureBox.Click event handler:

void myPictureBox_Click(object sender, EventArgs e)
    {
    Rectangle screen = Screen.PrimaryScreen.WorkingArea;
    form2.Location = new Point((screen.Width - form2.Width) / 2, (screen.Height - form2.Height) / 2);
    }

[edit]Spelling mistake[/edit]