Asma, this is no small task and the question you're asking isn't really specific enough for this forum.
You can make use of libraries such as AForge.net
] which will have you on your way down this kind of development, but you'll have to do the heavy lifting on your own.
Facial recognition components might also help: you would be able to get bearings on where the face is in the picture. This might allow you to "extract" the face, then you could perhaps use that in combination with AForge to make your magic.
If you do, however, find a Imaging.HumanFaceManipulation library let me know!
When you start plowing through the deeper parts of the problem, and if you run into any specific questions, please do not hesitate to post them here.