If you click in the reply box, then click on the icon that looks like a picture, it says insert image, you can then paste the url to your flickr image in there.
Topper, when viewing the picture you want to post you click the box above it that has the mail/facebook/twitter icon. This will open a drop-down menu. Click the sub-menu that says "Grab the HTML/BBcode" and copy/paste the contents of that box into a post.