Credits to Ron Price (I sat in a couple of webinars he led and this was something he demonstrated) ... When you insert a marker, it has a set of default features (size/shape/color etc.). So you find whatever image or icon you want to make into a marker (in the example here, I searched for "lungs" in the icon library; I inserted it into the slide, then cut it so it was available on the clipboard.) Then open the default state of the marker and then change the fill and color of the marker to "no fill" (so it's essentially transparent), and paste in the image or icon. That basically turns the icon into a marker. Once the icon is the marker's default state, it's a simple process of changing colors to create a "visited" state.
Here's the source file - good luck!
