Using TImage's Palette

Hi......

Im a little stuck....

How do i change the values of a TBitmap palette?

Ive created a TImage on my main form (which contains a preloaded BMP) and
want to change the palette values to brighten or darken the picture.

Ive figured out the palette is contained TImage->Picture->Bitmap->Palette
but how do i use it and access the palette values.

Or is there a better and easier way to perform what im trying to do.

Thanks heaps,
    Mark Williams