asp.net VB set background image of div tag in code behind

Hi, I have a div tag and I want to set it's background image in my code behind file. How would I do this? Below is what I currently have, but it is not working. Thanks

Page:
<div class="image_bg" id="image_bg" runat="server">     <<<<<<<<<<<<<<< want to set this bg image
  <div id="menuwrapper">  
    <div id="menu">  
      <div class="menuitem">asdfasdfsf k jlkj kl; kl lkj l;k jlk jkl kl asdfasdfsf k jlkj kl; kl lkj l;k jlk jkl kl asdfasdfsf k jlkj kl; kl lkj l;k jlk jkl kl asdfasdfsf k jlkj kl; kl lkj l;k jlk jkl kl asdfasdfsf k jlkj kl; kl lkj l;k jlk jkl kl asdfasdfsf k jlkj kl; kl lkj l;k jlk jkl kl asdfasdfsf k jlkj kl; kl lkj l;k jlk jkl kl asdfasdfsf k jlkj kl; kl lkj l;k jlk jkl kl asdfasdfsf k jlkj kl; kl lkj l;k jlk jkl kl asdfasdfsf k jlkj kl; kl lkj l;k jlk jkl kl asdfasdfsf k jlkj kl; kl lkj l;k jlk jkl kl </div><!--Template for each menu item-->  
      </div>  
   </div>
</div>  

Code Behind:
Partial Class vertical_test
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        image_bg.background-image = "url(main_slides/slide1.jpg)")
    End Sub
End Class