Hi,
I think I am getting little bit lost here :-)
I am not following you on the
media libraries - for each site you can have separate library with a separate folder so the URL of the image will be different.
The next question depends on your URLs and SEO settings whether you require the trailing slash or not. The resizing parameter is quite important there, but I think that after some customization it could be removed.
And the last one - you can get all the attachment data using the API and then it is up to you how you will compose the URL markup with all the parameters you need and want.
Best regards,
Juraj Ondrus