It cannot be done in the photography forum. In this forum, you cannot have pics displayed in a post that are hosted remotely. The only way to get a pic to show in a post is to attach it, and the boards only allow 1 attachment per post.
In the other forums, you can post pics that are hosted on different sites. So you can post multiple pics in 1 post (like you saw on the reef board).
The mods have done it this way to keep the bandwith/page sizes reasonable in the photography forum.
At least, that's the way I understand things - someone correct me if I'm wrong.