Personally I use Photobucket, though I have a Facebook account that I don't use much. If you use Facebook you need to find out how to find the URL of the photo. Many of us like PB because it is easy to obtain that address.
In any case, when you have the address the easiest way to proceed from there is to use the "rich formatting" option. In the icon bar above the composition window there is an icon representing a single frame of film. Click on that, paste the URL in the resulting window, and click INSERT.