If the background doesn’t have to repeat, you can use the sprite technique (sliding-doors) where you put all the images with differing opacity into one (next to each other) and then just shift them around with
Or you could declare the same partially transparent background image more than once, if your target browser supports multiple backgrounds (Firefox 3.6+, Safari 1.0+, Chrome 1.3+, Opera 10.5+, Internet Explorer 9+). The opacity of those multiple images should add up, the more backgrounds you define.
This process of combining transparencies is called Alpha Blending and calculated as (thanks @IainFraser):
αᵣ = α₁ + α₂(1-α₁) where
α ranges between 0 and 1.