I'd assume it has to do with bandwith, as mentioned earlier. Linking a pic from one site to post to another takes bandwidth away from the host site. Linking a pic to a high-traffic site, such as pirate4x4 is a good way to suck a hell of a lot of bandwidth, which we all know isn't free. At least that's my understanding of it...
I use a free account on one of those web image hosting sites and with the few pics that I have linked over on pirate, I've used 517.60mb of my 500mb monthly bandwidth allowed... oopsie