If you implement a Media Provider to load in the external images, then you can apply all the resizing using /media(...)?h=100&action=crop since the mediahandler basically wouldn't know if the image comes from Twitter, FB or the C1 Media Archive.
Look here how to create a mediaprovider
Look here how to create a mediaprovider
- https://bitbucket.org/burningice/compositec1contrib/src/502f68a9d7ba4b92f1ba09e031ee3eb12c005e1c/DataProviders.FacebookMediaProvider/?at=default
- https://bitbucket.org/burningice/compositec1contrib/src/502f68a9d7ba4b92f1ba09e031ee3eb12c005e1c/DataProviders.FacebookMediaProvider/FacebookFileStreamManager.cs?at=default