For those of you using Firefox, there's a brilliant solution to the problems many report about not finding the URL needed to post pictures after the launch of the new flickr version. First, get Greasemonkey - you won't regret this, as there are soooo many GM powered userscripts around to make life easier! Next, get one of those userscripts, the BBCode Creator.
Now, when you click the "Share this" option over each photo in flickr, then "Grab the HTML", you will find not only the HTML but also the BBCode you'll need, for all sizes.
(updated as flickr have added the BBCode option to the dropdown, you won't need to get this particular userscript anymore to have the forum code served - I still recommend getting Greasemonkey and browse the
Flickr Hacks group for neat and helpful userscripts)