I am not real clear on exactly what you want to do. However, one suggestion I have for you is to look at this:
https://github.com/xamarin/prebuilt-apps/tree/master/FieldService
This is a sample application that was built by Xamarin that has a nice login screen that you could try and get some ideas from. It does not use Monotouch.Dialog for the login screen, so if you want that you can look at this:
Your code is similar to that, but it looks like you modified it a little.
If you want to change your Login StringElement, you need to modify it, then call Root.Reload(login, UITableViewRowAnimation.Automatic);
afterwards. I don't think there is a simple way to just change the background image of a StringElement. You will need to make your own subclass of StringElement and customize the GetCell()
routine to. You can refer to this SO question for more ideas on that:
How can I create a custom styled EntryElement with MonoTouch.dialog?
I hope that helps.