Well, I'll just point out that you can _link_ to a picture just fine. People can click the link and look at it, so I don't think it really harms _discussion_.
I can't imagine the logo contest discussion we had on the French Wikipedia without the possibility to show pictures on discuss pages (unless copying all files on server).
On the other hand, we do end up with people uploading pictures of their cats/buddies/etc to stick on their user pages, which clutters things up. On the other other hand, many people don't have (or don't know how to use) a web-accessible space where they can upload pictures and then link to them, and they'd likely continue using the wiki's upload space for this stuff.
So what to do?
If copying non encyclopedic pictures on the server is not a problem we effectively don't need to be able to show external pictures on talk/user/meta pages. In fact, I suppose it is not really difficult to only add to a static version (ex: DVD) only files linked in articles.
Enabling external pictures pose the slight privacy risk of exposing people to 'web bugs' so an external site can track which IP addresses have visited a certain page, though perhaps this is minor.
Good point. Personally I don't care to be tracked, but if you think it's important, I agree.
-- brion vibber (brion @ pobox.com)
Thank to your answer.