You can use a simple bool value.
bool image1 = true;
private void BtnPicBoxDCS_OnMouseDoubleClick(object sender, EventArgs e)
{
BtnPicBoxDCS.Image = this.image1 ? global::CvSharp.Properties.Resources._035 : global::CvSharp.Properties.Resources._038;
this.image1 = this.image1 ? false : true;
}