You try again.
While Greg's page is not the easiest read, that is how you do it.
It works in Word 2016 the same way it does in Word 2007, 2010, and 2013.
Modify Quick Access Toolbar (QAT) Button Image or Text

If you want custom icons that are not used in Office at all, I believe you'll need to create a custom Ribbon Tab accessing your macros. Then transfer the buttons with the custom images to the QAT. If all you want is one of the icons available in Office then you can use the msoID.

Modifying the QAT in Microsoft Word