Image Rasterization..

Can anybody tell me, how to raster an image?
  Our project is about graphical password authentication using cued click points....
In this we r going to provide a series of images.. User has to click at right spots on each image. If clicks at rite spot on 1st image then 2nd correct image will b appeared.
If user clicks at wrong location then an image will appeared which is not in correct sequence... Thus, user has to click at right spots on each image...
 If he clicks at right spot on 1st image. then 2nd correct image will be appeared..
In this way user has to click at right spots on each image to crack the password..
This is an alternative to textual passwords for d sake of security......
      So guide us, how we can proceed & and is it possible to simulate this concept in C#.NET & how?.... plz guide us......