How do I can Pixels in a Raw Image?

I've got a sample skeleton program in C and have to change a loaded grey raw image to increase the bightness, contast etc.

I do not know how to edit the pixels that show on the screen as i've not used C to do this before. The program follows and I'd appreciate if someone can do a simple changing of all the pixels to say black or 0 value on one of the spaces for functions that are already set up there.

I'm in somewhat of a rush and help is appreciated :)