How to put a .png with a alpha channel as a texture of a cube?

Hi guys. I'm doing a mapping on a cube. The picture attached is what i want to do.

My plan is to add a .png as a texture onto the outer cube, so we can see the inner one.

Here is the question,
1) I can't add the png as a texture. Jpg works, but it doesn't have a alpha channel.
2) How to put an additional cube into the outer one?

I've looked into the texture tutorial. i can't load a picture with a alpha channel.

Any suggestion on these?