I remember there was an example in one of the Max 4 Help Files, where there was a yellow smiley-face button, you clicked it and a bullet-hole appeared in the forehead. very cute. the point is that it was an image with an image mask, so only the visible (circular) part was clickable.

I think it’s[pictctrl], you can specify that it has an image mask, supply the mask in the proper format (see help file as it needs to be exact). Then you can click just the non-rectangular area.