A clean solution for adding _target attribute on flickr thumbnails
There isn't one - links contained in HTML in a node body are not structured, do not run through l(), and no opportunity is given to 'hook' into them during rendering.

You have 3 options:


Change the markup to include the target attribute.
Preprocess the field and run a regex replace on the HTML to add the attribute.
Use javascript (by far the best option).