Using shape outline as mask

Hi, 

I'm all new to Animate, so maybe this is very basic. However I've been searching around and made lots of diffeent test - but with no luck.

I'm trying to create a ripple wave going from center of my image and out.

I've tried to animate a circle outline but though it looks like it's working the result of the mask is quite different. As such it looks like the outline goes from the center and out. It simply scales from small and then at last fram it's so big it's scaled out of the picture.

But when I combine the outline mask with the background image I can see that the mask is not only the outline but acutally covers the entire circle. So in effect I might have just animated an expanding circle where I was trying to animate only the outline to make it look like a ripple wave going over my picture.

Hope this makes sense. Quite hard to explain without writing a LOOOOOG post.

Maybe if someone could point me to a tutorial where someone has an animated outline that works as a masks that would help.

Thanks