List of built-in icons
Users should be able to use build in icons for custom buttons. Or add a placeholder icon at least.
Kevin Goodspeed commented
I'd like to see this as well. While I was able to copy & paste the icon path of a button that's already on the ribbon/command bar, I can't see the icons for buttons that are not on it.
You can always select any existing built-in button on a ribbon and copy the image path from the properties and paste that into your custom button image properties.
Would that work for you?