Basically, don't.
Instead pass the Image that the PictureBox is displaying as a parameter to the Constructor of the new form:
public partial class FrmItemDetail : Form
{
public FrmItemDetail(Image pic)
{
MyPictureBox.Image = pic;
}
...
}
Then just pass it the relevant picture:
private void ShowItemDetail()
{
FrmItemDetail frm = new FrmItemDetail(PicBoxItem.Image);
frm.ShowDialog();
}
That way, the internals of the new form are unknown to the "parent" so it can do what it needs to and change when it must without affecting the outside world.