Hi,
Good question, but tricky case
This is a feature, not a bug and is handled by Unity itself. These actions only use the Unity api, so what it returns isn't something we can do anything about.
http://answers.unity3d.com/questions/850739/click-through-transparent-parts-of-image.htmlso basically, clicks only happen with colliders, if you have a complex shape, you'll need to either get the pixel under the click, check for it's alpha and forward only if not fully transparent, or add a mesh collider defining the clickable area.
Bye,
Jean