private void sizeItem_Popup(object sender, System.EventArgs e)
{
normalSizeItem.Checked = (pictureBox.SizeMode == PictureBoxSizeMode.Normal);
stretchedSizeItem.Checked = (pictureBox.SizeMode == PictureBoxSizeMode.StretchImage);
centeredSizeItem.Checked = (pictureBox.SizeMode == PictureBoxSizeMode.CenterImage);
}