I'm trying to embed a Flickr photo into a weblog post.
Yea! It looks like I've been successful!
2014-03-01 Update: I've figured out some things and am adding them here as a reminder. To embed a Flickr photo such that clicking on it brings up the Flickr page in a new tab, use the SquareSpace Markdown block, use the code from Flickr's Grab the HTML/BBCode sharing method, and add target="_blank" after the href URL in the HTML tag.
Here's the code I used for this post:
<a href="http://www.flickr.com/photos/swolfe/11978832104/" title="Dawn in Eugene, Oregon (Explored) by Stephen A. Wolfe, on Flickr"><img src="https://v4s2.yimg.com/sk/3692/11978832104_45720ecd18_o.jpg" width="2048" height="1365" alt="Dawn in Eugene, Oregon (Explored)"></a>