On the other hand, if you end up hosting more image data at your end - and the site keeps growing (as I'm sure it will) then I'm concerned that a point will get reached (sooner rather than later) where we'll start to see performance issues - upgrades & more bandwidth will be required - and then we start getting into the territory of how that needs to be funded - and (heaven forbid) subscriptions and/or advertising may be required.
In my opinion, inline posting from external sources must shift the majority of the bandwidth burdon to the users and their chosen hosting sites; or "someone elses problem" from a Cambridge in Colour bandwidth perspective.