Is it possible to use a black and white image as a mask to another image the same size. If so, is an alpha channel or a mask the technique to use?

not sure what you're trying achive... can you post an example of sorts??

they say a picture is worth a thousands words.. especially when it's an explanation ;)

:)
retrocity

Cobra, yes and yes.

This is just an example. I hope this conveys the idea.

I want the blue layer to have a transparent gradient round hole so it shows the round picture underneath it. How to I use the black and white image to make that effect?

So far I get strange results.

Create a new image the same size as your blue and b/w images. Place your round picture in the center. Place the blue image as a new layer over your round picture. Create a new Alpha Channel. Copy the black and white image. Paste the black and white image into your new Alpha Channel. Ctrl-select your Alpha Channel so that you get the crawling ants. Go to your blue layer. Select Layer > Add Layer Mask > Hide Selection.

Crawling ants?? LOVE it...... that's the best description yet. Lou.

"..... and that was when things got interestiing."

Yep. I wish I'd thought of it first, though. :)

That worked. So just to clarify: I am using the alpha channel as a mask by following these steps? Thanks

Actually, in this case, it's more like a holding area. I just had you use the alpha channel because it's easier to paste the black and white image to it and so you could more easily make a selection with the channel before creating the actual mask. The mask, on the other hand, is on the layer itself. It's kind of like an alpha channel that belongs only to that layer and that goes wherever the layer goes. :)

Crawling ants! That's funny - never thought of it that way! -Lew ;-)