On Flickr click on the all sizes button for the photo you want, select the size, and then at the bottom of that page copy option #2 (option 1 that they tell you to use does not work on the DIS).
Then come here, make a post, click the yellow square with the mountain and moon on it near the bold, italics, underline buttons, past your Flickr link into that, and click ok. Done. If you really want to meet Flickr guidelines you then also copy the Flickr url for your photo, then copy your DIS pic link and click the world with the paper clip and paste your Flickr URL - your picture then becomes clickable and anyone clicking it will be taken to your Flickr page (which is how Flickr likes it).