fix bug in smbg exit selection

I just discovered a fairly nasty rendering bug from switching out images to circles for the smbgs - one remaining instance of `image` instead of `circle` that was preventing stale data from getting removed from the DOM.

The performance hit on this can be quite high if users scroll back and forth a lot, so I'd like to get this in ASAP. I'll merge tomorrow morning if no one objects, cut a new tag, and try to get it onto devel and staging quite soon.

Have it on my list to figure out a way to write a test for the exit selections (= DOM removal).