比较两个图像-error对象当前正在其他地方使用

我有一个大图像，我把它裁剪成小图像，并与打开的图像进行比较。它在没有线程的情况下正常运行。当我用线程运行时，它显示错误"Object当前正在其他地方使用。"

方法CropBitmap

        public Bitmap CropBitmap(Bitmap bitmap, int cropX, int cropY, int cropWidth, int cropHeight)
    {
        Rectangle rect = new Rectangle(cropX, cropY, cropWidth, cropHeight);
        Bitmap cropped = bitmap.Clone(rect, bitmap.PixelFormat);
        return cropped;
    }

方法比较Img

        public bool ImageCompareString(Bitmap firstImage, Bitmap secondImage)
    {
        int x, y;
        int count = 0;
        // Loop through the images pixels to reset color.
        for (x = 0; x < firstImage.Width; x++)
        {
            for (y = 0; y < firstImage.Height; y++)
            {
                Color pixelColor1 = firstImage.GetPixel(x, y);
                Color pixelColor2 = secondImage.GetPixel(x, y);
                if (pixelColor1 != pixelColor2)
                {
                    count++;
                }
            }
        }
        if (count > 400)
        {
            return false;
        }
        else
        {
            return true;
        }
    }

线程

        void compare()
    {
        Bitmap captchaFull = new Bitmap(@"C:'Documents and Settings'Administrator'My Documents'largeImg.bmp");
        Bitmap pic1 = new Bitmap(openFileDialog1.FileName);
        for (int x = 0; x < captchaFull.Width; x = x + 53)
        {
            for (int y = 0; y < captchaFull.Height; y = y + 44)
            {
                Bitmap temp = CropBitmap(captchaFull, x, y, 53, 44);
                pxImg2.Image = temp;
                if (ImageCompareString(temp, pic1))
                {
                    pxImg2.Image = temp;
                    return;
                }
            }
        }
    }

更新1:

                    Image check;
                lock (pxImg2)
                {
                    check = pxImg2.Image;
                    if (ImageCompareString(pic1, new Bitmap(check)))
                    {
                        pxImg2.Image = new Bitmap(check);
                        return;
                    }
                }

我锁定pxImg2，但它仍然错误它在Application.Run（new Form1（））的其他行中警告错误；

Image DummyImage;
// Paint
lock (DummyImage)
     e.Graphics.DrawImage(DummyImage, 10, 10);
// Access Image properties
Size ImageSize;
lock (DummyImage)
   ImageSize = DummyImage.Size;