Is it already hosted on the web? If so, just click the IMG button in the toolbar of the posting window, and put in the full web address of the image. Some free hosts don't allow off-site links, though.
If it a picture you have locally on your hard drive, you have to ATTACH it, which will upload it to Jordan's server. There are size limits but I do not know them offhand.
Hope this helps, and welcome aboard!