如何旋转或者翻转图片？

用Image.RotateFlip方法可以旋转图片90/180/270度或者垂直/水平翻转图片。

RotateFlip的参数是System.Drawing.RotateFlipType，该参数指定了提高给图片的旋转和翻转的类型。

代码片段：
//指定了无翻转的90度旋转
pictureBox1.Image.RotateFlip(RotateFlipType.Rotate180FlipNone);
//指定了无旋转的水平翻转
pictureBox1.Image.RotateFlip(RotateFlipType.RotateNoneFlipX);
//指定了90度的旋转，然后水平翻转
pictureBox1.Image.RotateFlip(RotateFlipType.Rotate90FlipXY);

如果想要以任意角度旋转图片，可以使用System.Drawing.Drawing2D.Matrix
代码片段：
public static Bitmap RotateImg(Bitmap bmp, float angle, Color bkColor)
{
int w = bmp.Width;
int h = bmp.Height;
PixelFormat pf = default(PixelFormat);
if (bkColor == Color.Transparent)
{
pf = PixelFormat.Format32bppArgb;
}
else
{
pf = bmp.PixelFormat;
}

Bitmap tempImg = new Bitmap(w, h, pf);
Graphics g = Graphics.FromImage(tempImg);
g.Clear(bkColor);
g.DrawImageUnscaled(bmp, 1, 1);
g.Dispose();

GraphicsPath path = new GraphicsPath();
path.AddRectangle(new RectangleF(0f, 0f, w, h));
Matrix mtrx = new Matrix();
//用System.Drawing.Drawing2D.Matrix类
mtrx.Rotate(angle);
RectangleF rct = path.GetBounds(mtrx);
Bitmap newImg = new Bitmap(Convert.ToInt32(rct.Width), Convert.ToInt32(rct.Height), pf);
g = Graphics.FromImage(newImg);
g.Clear(bkColor);
g.TranslateTransform(-rct.X, -rct.Y);
g.RotateTransform(angle);
g.InterpolationMode = InterpolationMode.HighQualityBilinear;
g.DrawImageUnscaled(tempImg, 0, 0);
g.Dispose();
tempImg.Dispose();
return newImg;
}
public Form1()
{
InitializeComponent();
pictureBox1.SizeMode = PictureBoxSizeMode.AutoSize;
pictureBox1.Image = Image.FromFile(@"C:\image.jpg");
pictureBox1.SizeMode = PictureBoxSizeMode.AutoSize;
Bitmap bitmap = (Bitmap)pictureBox1.Image;
pictureBox1.Image = (Image)(RotateImg(bitmap, 30.0f, Color.Transparent));
}