Q:

How do I preserve transparency over a 'paste' operation in photoshop?

Basic situation: 
I have an image with a complex alpha channel (ie. feathered, complex non-linear edges). 
I have an image I want to overlay on top of the first image using a blend option like multiply or maybe screen.
...but I must preserve the EXACT alpha channel of first image.
Can I do this in photoshop?
If I paste the second image it creates a new layer, and the multiply, screen etc options overwrite the alpha values on the first image (layer 1).
Seems like there should be an easy way to do this?
I saw this question:
How do I change RGB value without changing the Alpha channel?
...but messing around with the lock-alpha options didn't seem to do anything.
NB. The reason I mention the complex edges in the image is that the feature tool doesn't select the alpha channel correctly; simply select-using-magic on layer 1, goto layer 2, delete doesn't even remotely work.

A:

Simply use a clipping mask within Photoshop.
Option (mac) or Alt (Win) click between two layers. The Top layer will then be restricted to only displaying in areas where the bottom layer exists.

