Hi,
You can find an inspiration in the following code. This code creates an relative URL path to the image.       
  
        
       Guid iguid = new Guid("98de82d1-5eec-46cd-b491-42a31f31e1d2");
       
        // Tree node
        CMS.TreeEngine.TreeNode node = null;
      
        // Tree provider
        UserInfo ui = UserInfoProvider.GetUserInfo("administrator");
        CMS.TreeEngine.TreeProvider tree = new CMS.TreeEngine.TreeProvider(ui);
        // Get Single node specified by it`s GUID, culture code, site code
        node = tree.SelectSingleNode(iguid, "en-US", "CorporateSite");
        image.ImageUrl = "/Virtual_server_name_if_you_use_some"+ node.NodeAliasPath + ".aspx";
        
Best regards,
Ivana Tomanickova