Setting mainform data from another form's data

Assuming that 'mainform' instanciates the 'otherform' then you should raise an event.

There is an example in this tip.[^]. It uses a string value but you can easily alter it to pass a bool to set the Visible property of the PictureBox.

[edit] OK - so why the 1 vote? Accurate information that follows OOP best practices along with a link to example code, along with an explanation below on why the advice given previously was not great. What more could I do to help the OP? [edit]

Not sure how you're creating the forms, but let's say that the main form is created first and the independent form is created from the main form. So, in your independent form declaration, pass in the main form so that you can access it's properties

IndependentForm form2 = New IndependentForm(this);
form2.Show();

The IndependentForm would look like

public partial class IndependentForm : Form
{
  MainForm _parent;
  Boolean _pictureBoxVisble;

  public IndependentForm(MainForm parent)
  {
    _parent = parent;
    _pictureBoxVisible = false;
  }

  public void Button1_Click(object sender, EventArgs e)
  {
    _pictureBoxVisble = !_pictureBoxVisible;
    _parent.SetPictureBoxVisible(_pictureBoxVisible);
  }
}

Then, when you click the button on the independent form, you have the reference to the initial form and you can call a property or function on the main form.

mainForm.SetPictureBoxVisible(TrueOrFalseValue);