Wow, they really don't want to make it easy do they. I've tried various things, the one they want you to use is:
Go to flickr and click on the photo, in the bottom right is a bent arrow pointing right and if you hover over it it says "Share". Click on that and box pops up, if you click "BB Code" you can copy what is in the bottom box and paste it directly in here. The problem with this is the max size you get is 320 and it expects people to click the image to see the bigger version on flickr.
An alternative is to click the arrow on the far right "Download all sizes" then click "View All Sizes" select the size you want and then right click on the image and depending on your browser choose "copy image location" or "image properties" and get the url of the image and paste between the img tags
Cross psoted with cap'n chris but gives an alternative