To get a certian pixel, use the GetPixel method. To Set the pixel, you use the SetPixel method. Then when you get the pixel, convert it into RGB and Invert the numbers.

Say, for example, if the picture is 50x100...

Code:
For i = 1 to 50
   For o = 1 to 100
    Retval = GetPixel(Picture1.hDC, i, o)
    SetPixel pictur1.hdc, i, o, RGB(red, green, blue)
   Next o
Next i