1-Attach image to a post using the 'attachment' controls (bottom of this page)
2-Click 'Post Message'
3-Click on the 'Download' button for the image in your new message
4-Copy the address bar from the new explorer that pops up
5-Click 'Edit' in your message
6-Click the 'Image' button and paste your link where it says 'null' IE. [image.]null[/image.]
7-Click 'Edit Message' and your image should now be visible in the edited message
It could be you are using the wrong link from your host (look for "Direct link", then you could use your image host). I prefer to use attachements here as they stay on the site even if your image host loses your images...