System.Drawing.Image image = System.Drawing.Image.FromFile("c:\\mypic.jpg");
System.Drawing.Image.GetThumbnailImageAbort tnabort = new System.Drawing.Image.GetThumbnailImageAbort(tnCallback);
System.Drawing.Image tnimage = image.GetThumbnailImage(32, 32, tnabort, IntPtr.Zero);
tnimage.Save("c:\\tnmypic.jpg");

public bool tnCallback()
{
        return true;
}