Tap the pencil button in the top right of the main view to go into edit mode. Tap on the action you want to set an icon for. Next tap the “Icon” row under Options. Now set up a custom icon with the glyph you want to show up in Quick Actions. Once the glyph has been selected, you can switch back to using the Default icon or a photo icon if that's preferable. Now just save the action and you'll see an icon when you use Quick Actions.