Flickr is much more simple than that. On the top left side by the picture there is a "Share" button click on that select BBcode copy this and then paste it in to the forum having selected the size you wish. On this forum they show up at 800 on the longest side.
I'm aware of that but it doesn't work for me. It works in other forums but not this one. Here is the BB code to one of my photos that I copied from Flickr and pasted directly onto this reply:
<a href="http://www.flickr.com/photos/chuckabutty/8294663505/" title="Love Creek 8 by The Only Bill Smith, on Flickr"><img src="http://farm9.staticflickr.com/8362/8294663505_461342915c_c.jpg" width="800" height="530" alt="Love Creek 8"></a>