StencylForge has been discontinued for a while, all the behaviors that are still there are pretty old and not exactly reliable.
You can have a behavior that checks for the actor's image, and only detects non transparent pixels as valid for a mouse check on actor. Alternatively, create a system with scene/actor behaviors that gets all the actors that were clicked on, with a custom block that returns the actor in the top layer/highest Z-order, which ever is in front, to circumvent the overlap thing.