The details in your question are a little vague:
a. You don't state what element the image is the background of. (html, body, #story, #passages, .passage, etc...)
b. You don't state if the image is already sized to fit the default dimensions of the page or if it needs scaling.
c. You don't state how or where you want to display the "transparent text box" (whole passage are, part of it, etc..)
Place the following CSS example in your Story Stylesheet area, it makes the following assumption:
1. The image is the background of the html element and it is already sized to fit the default dimensions of the page.
2. The transparent text box is the whole of the passage area.
background-color: rgba(34, 34, 34, 0.6);
... the above uses the CSS rgba() function to allocate a alpha (transparency / opaque) value to the specified colour.