Find differences between images C#

thanks for hint. i really do not know about the usage of bitblt XOR for image compare & combine.

this is actually i want to do.after generating diff image from two picture how can i merge the diff image on the first picture. basically i am developing a screen sharing apps. only first time i will send full captured picture to other end and from the next time i will send only diff image comparing two images. so on the other i would like to merge diff image with last capture picture. so i am new in graphics program and one logic make me crazy that how can i merge small diff image with last image on the other end. if you know the logic then can u guide me with sample code. can u guide me how can i use bitblt XOR for image compare & combine. thanks a lot.