You can't add an image directly from your computer, it has to be uploaded to somewhere on the web already, like your Albums or Flickr.
You can then use the insert/edit image button to get it to show in your training log.
Well, it looks like I was wrong about Flickr. They hide image urls. Bastiges.
There's always Photobucket. They play much nicer than Flickr and even give you different versions of the URL.
Okay, I must be an idiot. I created a photo bucket account and tried to copy this link http://s195.photobucket.com/albums/z292/kproud/?action=view¤t=Lexi.jpg and load it but I still get nothing? Is this not what I am looking for?