Right now, you can only remotely load images on other sites if you use the site's built-in crossposter. So, since the built-in site crossposter only works to crosspost to LiveJournal and InsaneJournal, those are the only sites that you can remotely load images on.

It's possible that we may be able to lift the remote loading restriction in the future -- whether by site ("you can remotely load from these additional sites on the internet") or by account type ("images in paid and premium paid accounts can be remotely loaded from any site on the internet") or even by having some kind of add-on system where people can buy the right to remotely load their images from any site on the internet -- but we're being very very cautious with things for right now. Probably over-cautious, until we get more real-world data, but image hosting has the potential to eat up a metric fuckton of bandwidth and we don't want to incur huge bills.