We're definitely on the right track here. What little I know it this:
1. Post your pics to a photo website. (I use flickr, since I already have an account, but any will work.)
2. The photo website will provide a URL for each image. Copy this URL.
3. Paste the URL into your message. To have the image automatically appear (as opposed to the URL link only) use this exact format: [img]url-that-you-just-pasted[/img].
That sequence works for me, though my images don't come through full-width like some others.
I wonder if HikinJim might chime in with some tips. He posts some gorgeous photo essays.
BTW mods, are you still recommending a tiny image size?