Change image button image on client click

I have image buttons on my webpage with different images of normal hover and click I am using the folloing code below and it works fine for normal and hover but when i click the button i get an empty box with just submit in as if it can't find the image but its the correct url for the image and the image is there. 
<asp:ImageButton ID="btnAddUser" runat="server" Width="200" ImageUrl="/Images/buttons/btn_clickhere_normal.jpg" OnClientClick="this.src='/Images/buttons/btn_clickhere_clicked.jpg'" OnClick="btnAddUser_Click" onmouseover="this.src='/Images/buttons/btn_clickhere_hover.jpg'" onmouseout="this.src='/Images/buttons/btn_clickhere_normal.jpg'" /> protected void btnAddUser_Click(object sender, EventArgs e) { Response.Redirect("/adduser.aspx"); }