Create image with transparancy

//
// Loading source image.
// 

Aurigma.GraphicsMill.Bitmap image = 
	new Aurigma.GraphicsMill.Bitmap("x:/allformats/24bppRgb.jpg");

//
// In your case you will load mask from some external source.
// Just for demonstration I'll create it by myself here:
//
Aurigma.GraphicsMill.Bitmap mask = new Aurigma.GraphicsMill.Bitmap(
		image.Width, 
		image.Height, 
		Aurigma.GraphicsMill.PixelFormat.Format24bppRgb);
using (System.Drawing.Graphics g = mask.GetGdiplusGraphics())
	g.FillEllipse(
		System.Drawing.Brushes.White, 
		0, 
		0, 
		mask.Width, 
		mask.Height);

//
// Now we should just add alpha channel and assign mask into it.
// Please note, that mask should have the same dimensions, as image and
// it should be 8bppGrayscale.
//
mask.ColorManagement.Convert(
	Aurigma.GraphicsMill.PixelFormat.Format8bppGrayScale);
image.Channels.AddAlpha(0);
image.Channels[Aurigma.GraphicsMill.ColorChannel.Alpha] = mask;

// Thats all, saving the result.

image.Save("y:/maskedImage.png");