If you click the insert image link in the menu above the pane where you write your reply (little postcard with a mountain on it) you will get an a dialogue box come up.
Paste the URL of your FlickR image in the box & click OK, thwn it should be in your post - check by previewing first
Paste the URL of your FlickR image in the box & click OK, thwn it should be in your post - check by previewing first