creating a transparent gradient in a selection

I've discovered that I'm stuck.

What I want to do is make a row of five small thumbnail images that blend into each other. I've got them on five separate layers, and I just want to be able to select the edges of the pictures and create a horizontal gradient that makes the very outer edge almost 100% transparent, and the inner edge of my selection 100% opaque. Is there a way to do this?

ah. Nevermind, I discovered how to do it -- applying a layer mask and all that.