Hmm, I wonder if there is anything interesting you could do by creating a Service Worker for the CDN domain. It seems that the CDN will serve arbitrary files with a text/javascript Content-Type, which is the requirement to set one up. Once created, AFAIK, such a worker remains attached to the domain until an occasional attempt by the browser to refresh it fails, and it can intercept and modify any requests from webpages on its origin, as well as navigation there. But it doesn't affect requests to that origin from other origins, like the typical <img src> from facebook.com, so it won't be able to do much.
It seems that you click "download" on Facebook images, you'll get redirected to a CDN URL. So there's that. You could see what images the user is trying to download and edit them in flight... not a big deal, but it's something.
It seems that you click "download" on Facebook images, you'll get redirected to a CDN URL. So there's that. You could see what images the user is trying to download and edit them in flight... not a big deal, but it's something.