I'm looking for a efficient way to do image masking within Blender (using the masking tool) à la After Effects.  Basically, I want to cut out my images from within Blender instead of doing it destructively in an image editor.

Typically, in AE, I would precomp an image then do the masking in that precomp.  I'd like to mimic this workflow in Blender; however, it always seems overly complicated.  

I have considered creating a separate scene with the same dimensions as the image and masking there, then bringing it into a master scene to assemble in 3D space, but I'm not sure I can map this to a plane.

Sorry that I may not be explaining this very well. If anyone is familiar with AE, then you'll be familiar with what I'm trying to achieve.