I must admit to struggling a little to work out the correct process for this.
If you want to insert an image from your Flickr account to a post……
Create your new topic as normal, then in your flickr account, click the ‘share’ icon.
Then you’ll get the following form popup.
Make sure you click on Grab the HTML/BBCode (Items highlighted in pink)
Then copy ALL (Ctrl-A, Ctrl-C) text from the yellow highlighted part into your post.
Hope that helps