The most reliable way to work it out is to follow the logic Moodle will use.
If you get the URL of the image. For example, find the icon in the Moodle UI. Then (in Firefox) right-click and do View image properties.... it will look something like:
I put the key bits of that into bold.
You should be able to see the general rule for turning that into the type of path that Sam gave.
Moodle docs is a wiki, so you could add this explanation to Sam's document yourself, now that you understand it.