That's not really enough information for us to help you.
C++ (and C) don't do graphics. What libraries are you using? What do you mean by "image"? Stuff on disk? Something in memory?
ImageMagick is a good library solution if you don't already have something else going.
Sorry its my mistake i was not clear to you.. iam creating a paint application (like ms paint) using c program... using the graphics in c... i want to include a new option i.e color to grey scale any image in dos prompt has to be converted to greyscale.. can any one help..