Can I set background image and opacity in the same property?

I can see in CSS references how to set image transparency and how to set a background image. But how can I combine these two in order to set a transparent background image?

I have an image that I’d like to use as a background, but it is too bright – I’d like to turn the opacity down to about 0.2. How can I do this?

#main {
    background-image: url(/wp-content/uploads/2010/11/tandem.jpg); 
}