This plugin nowaday makes 2 mistakes in the XHTML 1.1 validation (and I think in others of W3C).
But the solution is really simply:
One is the use of target="_blank" in a label por links. That's not allowed. You just need to remove that or use another value to target.
The other one is the close slash in the img label. If you follow any XHTML recomendation, you need to close all of your labels. Just add "/" before ">" in the img markup and it's fine.