States for Images on Free Form Pick One Questions

A free form pick one question applies two states to image responses: Normal and Selected.

• Since the default Selected state is not visually different enough from the Normal state, I added a Dimmed state to the images.
• When a user selects their answer, all other images go to the Dimmed state so that the selected answer stands out.
• I have a trigger to set the state to Selected whenever a user clicks on an image (to select their answer). This enables them to select one answer, then another, etc. before they submit their final response.  
• However, when the Selected state is invoked in this way, it does not recolor the image. It stays grayed out just like my Dimmed state, even though the States Panel shows that the Selected state is in color.  (Screen captures attached.)
• The quiz question works fine--if I submit the question the selected answer is properly submitted, but the Selected answer is not "highlighted" for the user.

I cannot figure out why the images stay dimmed. Perhaps it's not kosher to mess with states in a quiz question? (I rarely use the built-in Quiz structures...but in this case I do need a scored Quiz.) Any guidance is appreciated.