Display not found image if the original image not found in ASP.Net

in my asp.net+vb web i am using this code to display image of workers in web page as per their id and it works fine

 Dim id1 As String = "11022"
            id1 = " + idtxt.Text + "
            Image1.ImageUrl = "~/photos/" + ID + ".jpg"
        End If

there persons whose photo is not uploaded in server . i want to show an alternate image name notfound.jpg if the photo is not found