@twigs the picture you post must already exist on another site, like Tinypic. You then reference its location by using the "picture" icon above the message window on FC. For example, say I want to post a picture from a website. I "right click" the image, choose the "image address", then paste that link into the picture icon's window above the message window on FC. If image is from a picture hosting site eg Tinypic, choose "Direct Link".
If that doesn't make sense, consider "Wake&Bake" status.