The problem is that Elements writes to the target attribute via the “Open in New Window” checkbox. So target is a reserved attribute that you can’t manually set.
We’d have to update Elements to allow you to set a custom target attribute — I’ll add a ticket for us to investigate this, however, I can’t say when we’ll be able to get to this one.
This would be important. Because sometimes you have an image link in the CMS that points to its own page (_self), and sometimes it’s an external link (_blank).
Perhaps the solution would be to specify “target” in the list of options, so that it takes priority and the clickable option isn’t used?