Change PixelFormat->JPEG's

how can i change the PixelFormat and Performance from JPEG's
then i want to save the pic with this lines(from Ken Moffat)

            TJPEGImage* J = new TJPEGImage;

i've made a screencapture and this is 24bit
i can save the bmp as jpg but i want change the PixelFormat(8bit) and

thx a lot