C++ :: Writing Image Data To Disk In Parallel?

Basically, I want to write an image to disk in parallel with an algorithm that is processing that same image. The code is really simple:

Code:
string ProcessImage(System::Drawing::Bitmap ^bmp, System::String^ targetFile) {
bmp->Save(targetFile);
System::Drawing::Bitmap^ bmp8 = BitmapConvertPixelFormat(bmp, 8); //<-- a function I wrote which converts the 32bpp I am passed into an 8bpp one
string results = Analyze(bmp8); //<--- takes a good bit of time
return results;
}

What would be the fastest way to do this? Any libraries with a code sample. I would prefer one that is within .NET or native C++ just to avoid having to link into many different libraries.