At peak it handles 500+ lines of chat a second. That's quite a bit more than twitter. It also has around twice the number of peak users than Meebo rooms. So although it's certainly not a massive site, it has taken some thought to scale.

Having big images/videos wouldn't really make any difference to which language you choose. That's just static media files which are irrelevant.

I really don't see how having more images on the page would make it harder to scale :/ if only the site were as simple as serving up some images...