Since you're posting in the GDI+ section, I assume you do want to use GDI+ to alter the image. Here's a link to some sample code: ColorMatrix Basics - Simple Image Color Adjustment.
Doing the brightness is pretty simple, just alter the RGB values by the same amount using the 'translate' function at the provided link. Doing the contrast will be a bit more difficult since it entails lightening the lighter pixels and darkening the darker pixels (or vice versa).
Thanks for the link it helped and clarified a lot and as far as my being using GDI, I couldn't think of anything else except GDI+ and was trying for some time with color matrix. Is there any other alternative ways of accomplishing this and which could be the best way to achieve this... any suggestions will be appreciated.