Adding/Removing Default Halo around Multi-Choice Quiz Answers

Hi everyone-

I've run into an issue that I can't seem to figure out how to resolve. 

I created a custom multi-select multiple-choice quiz question where the answer choices are images.  The user needs to select the three correct images to get the question right.

I want to change the state of images so I user knows when an image is selected and when it isn't.  I noticed that when I imported the images, three of them "automatically" had both a Normal state and a Selected state; the selected state has a blue halo around the image.

However, the fourth image had only a Normal state after import.  I added a Selected state, but cannot determine how  to add the blue halo effect, nor can I figure out how to remove it from the other three (it is not a shadow effect, I've already tried that).

Any ideas?

Thanks,

Jeremy