Photoshop :: CS5 - How To Create Transparency Around Edges Of Image

I'm an beginner/intermediate Photoshop user, using CS5. I'm working on an image of two women, starting from an historical photograph. I selected, copied and pasted the central image of the 2 women onto a new layer which is otherwise transparent. I then created a copy of the original photograph's layer and applied a masking layer with a radial transparency gradient, so that when the original layer is hidden, the background appears to be radially faded behind the central image. So far, so good.
 
However, I'd like to apply an additional "dose" of transparency around the edges, so the image fades completely at the outermost edges. And I'd like more control over this additional fading than just applying another gradient.
 
I've seen a video of someone "painting" transparency onto an image, but I can't quite get my brain around it. From what I've read, it looks as though I should create another layer that will be an additional masking layer... but how to do this.
 
My undying gratitude for any assistance to get me past this roadblock.