Use
FindName
as shown
here.
for(int i = 1;i < 40;i++) {
StackPanel sp = dt.FindName("OuterPanel",cp) as StackPanel;
Image img = sp.FindName("Image" + i) as Image;
img.Source = new BitmapImage(new Uri("pack://application:,,,/{AssemblyName};component/Images/MyImage.png");
}