Assign src to html image

 public string ServerValue = String.Empty;

   protected void Button1_Click(object sender, EventArgs e)
    {
        

        ServerValue = "/images/logo.jpg";



       
    }

<img id="box"   src="<% =ServerValue %>" alt="htis"
       /><br />
    <asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />