changing color depth of image data

Hello,

I have a CGImageRef which i have created by:

CGImageRef screenShot = CGWindowListCreateImage(bound , kCGWindowListOptionOnScreenOnly, kCGNullWindowID, kCGWindowImageDefault );

This CGImageRef is now used to create NSBitmapImageRep:

NSBitmapImageRep *bitmapRep = [[NSBitmapImageRep alloc] initWithCGImage:cgImage];

data=[bitmapRep representationUsingType:NSJPEGFileType properties:[NSDictionary dictionaryWithObject:NSImageCompressionMethod forKey"compression"]];

Now i have the image data with me, i want change the color depth per pixel to 256 colors or 32 bit, How can i do that?

Thanks,
Ruhi.