I do that with the photo's so they show up in the thumbnail in the forum listing. The way I do it using my iPad is to copy the pic by holding on it till it brings up that prompt. Then I click on the insert pic icon in my post and paste the URL in the "from URL" tab in the box that comes up, and uncheck the option to Retrieve local...then save it and type my post.
Then save my post. Then click to edit my post and save again...and it works.