You should use a second TImageEnView to contain the visible area. Select visible area, then copy it using AssignSelTo(). Example:
var
ie: TImageEnView;
begin
ImageEnView1.SelectionBase := iesbClientArea;
ImageEnView1.Select(0, 0, ImageEnView1.ExtentX, ImageEnView1.ExtentY);
ie := TImageEnView.Create(nil);
try
ImageEnView1.AssignSelTo(ie);
ie.IO.DoPrintPreviewDialog();
finally
ie.Free();
end;