What do you mean by "matrix"? My understanding of a matrix is just a 2D array, which is pretty much what an image is by default. If you want the actual data, you can take a look at the
Bitmap class on MSDN, specifically the GetPixel and SetPixel methods. If you're in XNA and you're using a Texture2D object, I know you can get the data as an array of Color objects.
Anything else and you'll have to do some digging I'm afraid. Usually the MSDN documentation is sufficient to show you how to access the data the the image is comprised of.