You're working semi-backwards, I think. Make an image with semi-transparency, say, in photoshop, and thensave for the web as a PNG that can handle transparency.
I use it on my site,
www.headlinercreative.com. The black gradient is the same skinny image repeated-y on every page. The stripes are a small square tiled on a layer below it, so the color shows through the gradient.
