There is not one particular code or url because it hasn't seemed to work for any pic I was trying to insert in Flickr.
For instance, when I grab the embed code for this creative commons picture by Flickr user Rupert Ganzer and insert it into a post, I can insert it, but it disappears after saving the draft and no picture is inserted.
<iframe src="http://www.flickr.com/photos/80384851@N00/3592047683/in/photolist-6tqbCB-NdDFh-78KyFA-7Y5JY-2Nh4Q8-8ri1zr-fjVtg-dvqWZr-dGUpaY-gVfNaJ-93j457-Nwstc-bxbmQ6-i2Bj1c-dPt4bL-bE68mU-bE68CQ-atcasm-dcZckf-f8A5wc-fSyMEB-ik3jFu-btR1HM-7ThZSt-CfESw-4HeAW-dvcNAc-ixqs5m-rVNm3-ZnM8P-8bBbS8-9wbQNN-dm341m-6BTeDC-aDfBWg-6P9LtB-3SkgVc-e1iU37-fcSbf5-fcSaCS-7ifa3h-aPdi82-e1X2eA-57y4FA-9ctHxv-9baL15-aaJ7bC-6pc3pW-6shpx-7RRZAT-7ajQiw/player/" width="500" height="333" frameborder="0" allowfullscreen webkitallowfullscreen mozallowfullscreen oallowfullscreen msallowfullscreen></iframe>
The same holds when I try to insert the pic via url: http://flic.kr/p/6tqbCB. However, here, the image never appears on the "Insert from URL" page after putting the URL into the box. Then just the URL link (as text) shows up in the post.
I thought this might be an issue of the picture size I was choosing, but the same holds for other sizes.
I was, however, successful this AM as I pulled a link to a morguefile pic and inserted via URL. The URL is http://mrg.bz/juIHrn.
Because this URL worked I'm left thinking it's either a Flickr Issue or my issue with how I use Flickr.