Non rectangular trait

Can somebody please clarify to me how to use the non-rectangular trait?

I have added the figure (the scan of a miniature, PNG format) in the basic piece trait. Then I have loaded it again in the non rectangular trait, which is right after the basic piece trait. After doing this I see the non rectangular trait displaying the miniature in black on a white background.

However, when I start a game and click/right-click on the piece, it will behave as a normal rectangular piece. The miniature correctly shows the background around it (the invisible part works), but the click-right/click is accepted even on the invisible parts.

What am I doing wrong?

Ok, figured out that the problem is due to an always-active layer defined on the image, which covers most of the image.

Despite it being almost completely transparent it insists on being completely selectable/right-clickable. But placing it above the non-rectangular trait solved the issue.