You can post multiple pics in the same post by using the IMG and /IMG notation. You simply surround IMG with [ and ] and then copy the link to your picture from your flickr account. In your album you should have a button that says "actions", and if you click it you can view all sizes. From there you can choose which size pic you want to post.
I use photobucket instead of flickr, so someone may know an easier way, but...It would look like this, except you would replace < > with [ ]: