using (Bitmap bitmap = new Bitmap(bildAlt))
{
    Bitmap neu = bitmap.ScaleProportional(500, 500);
    neu.Save(bildNeu, codec, para);
    neu.Dispose();
    bitmap.Dispose();
}

Rect layoutRec = new Rect(0,0,int.MaxValue,int.MaxValue);
Bitmap neu = bitmap.ScaleProportional(500, 500);
for(int x =0; x<=bitmap.width; x++)
  for(int y =0; y<=bitmap.height; y++)
    if(bitmap.GetPixel(x,y) != Color.White){
      if(x<layoutRec.X)
        layoutRec.X = x;
      if(y<layoutRec.Y)
        layoutRec.Y = y;
      if(x>layoutRec.Width)
        layoutRec.Width = x;
      if(y>layoutRec.Height)
        layoutRec.Height = y;
    }
Rectangle cropRectangle = new Rectangle(0, 0, max.X - min.X, max.Y - min.Y);
Bitmap newBitmap = new Bitmap(cropRectangle.Width, cropRectangle.Height);
using (Graphics g = Graphics.FromImage(newBitmap)) {
  g.DrawImage(neu, 0, 0, cropRectangle, GraphicsUnit.Pixel);
  return newBitmap;
}

using (Bitmap bitmap = new Bitmap(bildAlt))
{
    Point min = new Point(bitmap.Width, bitmap.Height);
    Point max = new Point(0, 0);

    for (int x = 0; x < bitmap.Width; x++)
    {
        for (int y = 0; y < bitmap.Height; y++)
        {
            Color pixelCol = bitmap.GetPixel(x, y);

            if (!(pixelCol.R >= 250 && pixelCol.G >= 250 && pixelCol.B >= 250))
            {
                if (x < min.X) min.X = x;
                if (y < min.Y) min.Y = y;
                if (x > max.X) max.X = x;
                if (y > max.Y) max.Y = y;
            }
        }
    }

    Rectangle cropRectangle = new Rectangle(min.X, min.Y, max.X - min.X, max.Y - min.Y);
    Bitmap bitmap2 = new Bitmap(cropRectangle.Width, cropRectangle.Height);
    bitmap2.SetResolution(bitmap.HorizontalResolution, bitmap.VerticalResolution);
    Graphics g = Graphics.FromImage(bitmap2);
    g.DrawImage(bitmap, 0, 0, cropRectangle, GraphicsUnit.Pixel);

    bitmap2.Save(bildNeu, codec, para);
}