from Flickr use the links below the actual picture you wish to post (need to choose size in flickr first), eg the link which ends in .jpg rather than HTML code
I click on the pic I want, then choose the size, then copy the link underneath the pic & insert it between the IMG tags (above the reply box)
Giving
or if you use firefox, get greasemonkey & use Phorum, which creates a link back to your flickr photo which doesn't go against the T&C of flickr (you're supposed to link back to flickr if you post your pics elsewhere)
Using Firefox as your browser, with grease monkey enabled with the Phorumr script enabled you get another link under the pics in Flickr, which when copied into a forum post (no need for IMG tags) makes the pic a link back to the pic on Flickr
Using a Phorumr link copied from Flickr:
You can also rejig the link to goto the large pic on flickr instead of the original one: