var topLeftCorner = webBrowser.PointToScreen(new System.Windows.Point(0, 0));
var topLeftGdiPoint = new System.Drawing.Point((int)topLeftCorner.X, (int)topLeftCorner.Y);
var size = new System.Drawing.Size((int)webBrowser.ActualWidth, (int)webBrowser.ActualHeight);
var screenShot = new Drawing.Bitmap((int)webBrowser.ActualWidth, (int)webBrowser.ActualHeight);
using (var graphics = Drawing.Graphics.FromImage(screenShot)) { graphics.CopyFromScreen(topLeftGdiPoint, new System.Drawing.Point(), size, Drawing.CopyPixelOperation.SourceCopy); }
screenShot.Save(@"D:\screenshot.bmp");