Its very unfortunate that all the MSFT apps behave like "islands" and are not extensible nor do they provide documentation of their protocol API.
So the only way to accomplish what you want to do is to write your own photo app and make it extensible. It will be quickly adopted by users if they can use it with their favourite pic cloud service.
You could use a GreaseMonkey approach to let others provide "plugins" or "extensions" that are verified by you as the app developer for malicious code and then digitally signed. Your app would download those script files into its local app folder
on demand and load the code during runtime via <script> tags dynamically, given the digital signature matches the one you have bundled with your app. Of course this code would not be bytecode optimized by the JS Engine.
That's basically what the Bing SDK does. The code you embed into your app is merely a stub that downloads "modules" on your apps behalf and executes them.