Detach a mask from the original image

Hello everyone,

I just started using after effects and I can't find a solution to my first problem. Here is my 'issue':

Let's say I have a composition with a PNG image. I created a mask, dividing the picture in two parts. Now I'd like to animate these two parts separately, which means the original image would be like the parent of the mask, and the mask could be manipulated as if it was a picture.

Sorry for asking noobish questions but hey, we all started somewhere

Thanks for all your answers