First, upload the photo to a Web-based photo-sharing service (flickr, photobucket, etc). From the service, copy the photo's URL.
Then on the site, you can either click the "Image" icon in the mini-editor, and paste in the URL, or you can type in:
[ img ]http://www.url-of-the-photo.jpg[ /img ]
(omit the spaces in the brackets - those are there to keep the coding visible.)
1) make sure the URL has http:, not https: https won't show up.
2) if the photo is in an album online, make sure you've got the URL for just the photo, not the album. The .jpg (or .gif or .png) is a sign it's correct.