Find differences between images C#

thanks for answer. would u plzz tell me why did u work with 4 picture box?

after generating diff image from two picture how can i merge the diff image on the first picture. basically i developing a screen sharing apps. only first time i will send full captured picture 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 can u guide me with sample code. thanks a lot.