为了让大家知道下面内容是否是自己想要的，我先发效果图。

好了，那就开始贴代码了

以下为一个按钮的事件，为裁剪准备图片、裁剪路径、保存路径

复制代码 代码如下:
private void button1_Click(object sender, EventArgs e)
{
GraphicsPath path = new GraphicsPath();
Point[] p = {
new Point(424,244),
new Point(240,440),
new Point(340,552),
new Point(550,526),
new Point(478,366),
new Point(348,660),
new Point(424,244)
};
path.AddLines(p);
OpenFileDialog openFileDialog = new OpenFileDialog();

openFileDialog.Filter = @"Bitmap文件(*.bmp)|*.bmp|Jpeg文件(*.jpg)|*.jpg|所有合适文件(*.bmp,*.jpg)|*.bmp;*.jpg";

openFileDialog.FilterIndex = 3;

openFileDialog.RestoreDirectory = true;

if (DialogResult.OK == openFileDialog.ShowDialog())
{

Bitmap bit = (Bitmap)Bitmap.FromFile(openFileDialog.FileName, false);

Bitmap newBit = null;
BitmapCrop(bit, path, out newBit);
newBit.Save(@"C:\Users\Public\Pictures\Sample Pictures\沙漠22.jpg");
}
}

以下为裁剪的关键代码

复制代码 代码如下:
/// <summary>
/// 图片截图
/// </summary>
/// <param name="bitmap">原图</param>
/// <param name="path">裁剪路径</param>
/// <param name="outputBitmap">输出图</param>
/// <returns></returns>
public static Bitmap BitmapCrop(Bitmap bitmap, GraphicsPath path, out Bitmap outputBitmap)
{
RectangleF rect = path.GetBounds();
int left = (int)rect.Left;
int top = (int)rect.Top;
int width = (int)rect.Width;
int height = (int)rect.Height;
Bitmap image = (Bitmap)bitmap.Clone();
outputBitmap = new Bitmap(width, height);
for (int i = left; i < left + width; i++)
{
for (int j = top; j < top + height; j++)
{
//判断坐标是否在路径中
if (path.IsVisible(i, j))
{
//复制原图区域的像素到输出图片
outputBitmap.SetPixel(i - left, j - top, image.GetPixel(i, j));
//设置原图这部分区域为透明
image.SetPixel(i, j, Color.FromArgb(0, image.GetPixel(i, j)));
}
else
{
outputBitmap.SetPixel(i - left, j - top, Color.FromArgb(0, 255, 255, 255));
}
}
}
bitmap.Dispose();
return image;
}

有了这个前台只需要获取用户的鼠标路径传到后台就可以。