Kim: Here's how it works for me:
1. Open your Flickr account.
2. Navigate to a photo and click on it.
3. Go to the tool bar right above the photo and select "All Sizes".
4. When the Available Sizes page appears, select Medium Size.
5. When the medium size photo is showing, go to the choices below the photo and select the second one, "Grab the photo's URL." Select it (ctrl-A) and copy it (ctrl-C).
6. Open up a forum reply or new message.
7. Click on the Img button above the message content box; the characters img and /img in brackets will appear in the message box.
8. Put your cursor between the close bracket on img and the open bracket on /img. Paste (ctrl-V) the URL from step 5.
9. Check the preview to see that the photo appears.
Here's an example of the outcome: