Drag and Drop - Doesn't Ignore Empty Pixels

I'm using Storyline 360.  My interaction involves choosing which zone to go to by dragging the target to the appropriate location.  The zones are defined by circular distance from a central point.  However, Storyline is treating the empty pixels in my PNGs as if they were filled and using square drop zones.  This doesn't work because my desired drop target borders meet each other on a curve.  Is there any way to have Storyline ignore transparent pixels in drop targets?

If I have students click on the appropriate zone instead of dragging the object, will the empty pixels be ignored?

Thanks!