ImageButton - ImageUrl doesn't change

I need to swap an ImageButton's ImageUrl when the ImageButton is clicked. There are 3 ImageButtons. Each could have either "plus" or "minus" image (so two URLs). At the same time only one could be "plus". I've stuck with that images simple don't change. I debug the code and it works fine, sort of. I mean in the code ImageUrl changes however at the client side nothing is changed. Below there is my code:
public partial class TreatmentRealisation : System.Web.UI.Page { const string PlusImageUrl = "~/Content/newAdd.png"; const string MinusImageUrl = "~/Content/negative.png"; protected void Page_Load(object sender, EventArgs e) { } protected void ImageButton_AddReferral_Click(object sender, ImageClickEventArgs e) { ImageButton clikedImageButton = (ImageButton)sender; if (NavigationImageButton.Count().Equals(0)) { GetNavig ...