Marcelo,
Faça desta forma:
public partial class Form1 : Form
{
public static Form1 frmInstancia;
public Form1()
{
InitializeComponent();
frmInstancia = this;
}
public static Form1 GetForm()
{
return frmInstancia;
}
public void AtribuirImagem(string caminho)
{
pictureBox1.ImageLocation = caminho;
}
}
public partial class Form2 : Form
{
public Form2()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
Form1.GetForm().AtribuirImagem(pictureBox1.ImageLocation);
}
}