How do I create a spot the difference ungraded question?

You could use a Freeform Multiple Response question. Instead of hotspots, use transparent shapes. 

Shapes can have states, so you could also design a Selected state to indicate what's been clicked. For example, the Selected state could have a glow. 

• Note: Selected is a built-in state. An object with a Selected state will automatically toggle between Selected and Normal when it is clicked. 

Here's more info about Freeforms: Storyline 360: Converting an Existing Slide to a Freeform Interaction | Articulate - Community