I think that this is going to be a simple question to answer since I believe that I am missing a fundamental understanding on what format the alpha masks need to be in.
I'm having a heck of a time trying to create my own alpha mask from the following image. I want the outside of the box to be 100% transparent, the border to be opaque, and the inside to be slightly transparent:

I want to apply the mask to this image:

I have tried many different combinations of
• Dropping the depth of the alpha.png image.
• Removing the alpha of the alpha.png image.
• Converting the alpha.png image to another image format.
• Anything else I could find on forums that looked like it did what I wanted.
Can anyone let me know how I can convert the alpha.png image into a true alpha mask to use?
Thanks!