But your above screenshot does show it is added. Note Indigo doesn't add every image, just the base image. So the +true variant won't show in the popup but it will be selected based on the variable value (when viewed from a Web browser or Indigo Touch, but not from within the Control Page editor itself).
I was expecting all of my png files to show up in the pop up. And even when I got the base image to show up in the CP edit image I also expected the "true" image to show up in the edit page when the variable became true.
It's all pretty confusing as to what works when and where.