You will have to follow three steps:
- Create a difference (DIFF) of the two consecutive images. Comparing the two images pixel per pixel will be very time consuming. You should utilize a well-established library like OpenCV; check out the Emgu CV library (http://www.emgu.com/) for .NET: AbsDiff() should be the method you're looking for. The result will be something like DIFF = IMG2 - IMG1.
- Send the DIFF through the network. It's still a full image, but JPEG or PNG will utilize its full compression capability assuming it is a mainly black image, i.e. few changes. So these are actually three substeps: Compress - Send - Decompress.
- Apply the DIFF on the present image. The recipient can calculate the next image IMG2 = DIFF + IMG1. This can be performed using EmguCV's Add method.