Sequential PrintScreens Comparison, Generate a difference image?

i am developing a application that save a printscreen at a regular interval, let's say 10 seconds.

In general the images are very similar, sometimes equal, so i came with the idea to create a bitmap that represents the difference between the current printscreen and the previous one.

To achieve this, i am comparing the 2 images, pixel by pixel, and when they are equal, i am setting the pixel with a Transparent Color (in the original code, i am using Bitmap.LockBits for a better performance):

for (var x = 0; x < width; y++)
for (var y = 0; y < height; y++)
{
   var oldColor = lastPrint.GetPixel(x, y);
   var color = currentPrint.GetPixel(x, y);

   if (oldColor == color)
   {
        differencePrint.SetPixel(x, y, Color.Transparent);
   }
}

To recover the image, i get the first printscreen and replace with the sequential bitmaps.

private void MergePrints()
    {
        var lastBitmap = new BitMap(firstPrint);
        foreach (var print in prints.OrderBy(e => e.Date))
        {
            using (var difference = new Bitmap(print.Image))
            {
                 using (var grfx = Graphics.FromImage(lastBitmap))
                 {
                     grfx.DrawImage(difference, 0, 0);
                 }                 
            }

            lastBitmap.Save(print.id + ".png");
        }
        lastBitmap.Dispose();
    }

My question is: Is there a better way to generate a object that represents the difference between the 2 images, other than a new image with transparent pixels? Maybe a new class? but this class need to be persisted and of course "smaller" than a bitmap, currently i am persisting the bitmap as byte[].