Textures' alpha not showing on display or BI render

Can’t for the life of me figure out how to get alpha channel to work.
I’ve read the entire chapter on materials in the manual, and nothing.
Trying to model a membrane switch, --printed metal wires on a transparent, flexible membrane. I’ve set up diffuse and specular textures. The wires are dark in the diffuse texture; light in the specular, as they are metal, but where there’s no wires the alpha goes to zero. The membrane goes on top of a red plastic thing I modeled also.
Both images have the Use Alpha setting.
Image sampling Alpha = Use.
My membrane material has Face Textures and Face Textures Alpha on.
The plastic thing has the receive transparent shadows setting on.
In the Scene panel, I have Combined only; nothing excluded.
In the render panel, file extensions is RGBA, not sure what it means.

EDIT:
In the Materials panel, there’s diffuse and specular colors with Intensity sliders, and I expected them to make no difference as I’m presumably using the textures, and making them black (from white) made no difference, but putting the intensity to zero my membrane looks black.
Pretty disconcerting; I don’t know what these things do, or how to get rid of their influence.
How do I use textures only?

Rather than have everyone having to try and decipher what you have written, why not just supply a demo blend file and I’m sure someone will tell you what you need to do to show the alpha. At the moment we know very little about your particular material and even less about the actual textures being used.