How can I use the black parts of an image as an opacity map? In other words, make all the white visible and the black invisible. Thanks.
Are you referring to the compositor or materials? Either way, they can both be done in similar ways. You can grab almost any output and plug it into any input to do all sorts of things. You could take the colour data of any image and use it as the data for some sort of alpha node.
For the compositor, how about this?
The colour ramp just clips both extremes so there are more defined boundaries between light and dark (similar to setting contrast but with more customisability)
For a material, try this:
As you can see in both examples, I am just taking the colour data as it is. No need to do anything complicated as the brightness of the image comes with the colour data and is what is read by any factor node.