this.pictureBox1.Image.RotateFlip(RotateFlipType.Rotate90FlipNone);

this.pictureBox1.Refresh();
逆时针旋转图片

this.pictureBox1.Image.RotateFlip(RotateFlipType.Rotate270FlipNone);
this.pictureBox1.Refresh();

this.pictureBox4.Image.RotateFlip(RotateFlipType.RotateNoneFlipX);  //左右镜像

还有180，上下镜像等