For the XML issue it looks like you're missing the opening quote in the src
tags, try
<image id=""1"" src=""" + MediaControl.AlbumArt + @"""/>
As for updating on a minute or 30-second basis, take a look at this sample on dev.windows.com.
Clearing the tile on termination could be tricky since you're not informed programmatically of all cases where the app is indeed terminated.