Changing the Opacity of a Background Image

Hi All,

I am using Harlowe 2.0.1 and am unsure how to change the opacity of a background image on my start page. This is what I have in the stylesheet:

html.start tw-story {background-image: url ("my image url"); background-size: cover;}

The image shows up fine, but I'm not sure what to add to change the opacity of the background image (without affecting the rest of the body). Can anyone help?

Thanks!