Using an external gif as a mask, not working properly

I want to use a gif as a mask. Normally this is fine, but this gif is funnily shaped and has a transparent background and i want the mask to be the funny shape. The problem is when i add the image as the mask, the mask is the rectangular shape of the image plus the background and not the actual shape. Any thoughts on how get this working?
thanks
dai2
:eye: