I have a GreaseMonkey extention that puts the appropriate BBCode on the "all sizes" page, but if you don't have that, here's how:
Go to "all sizes", select "medium". Go to the bottom and grab the picture location, not the HTML. Click
on your Cellar post, and put in your location. Then go back to the flickr picture page and grab the URL. On your Cellar post, highlight the picture, and click
, and put in the URL.