How to assign an existing icon image file to a button?

[Svengali]

For a long time I've wondered Is there some way to load an existing icon image file to a button?

Currently, in the Button Editor, there are three options for assigning an image/icon to a button:
1. choose from a list (a picture grid of current icons assigned to other buttons)
2. assign the current cutbrush to the button
3. assign the APen color to the button

I can't find anywhere how to assign some file (containing an image) to a button?

In practice, it seems that the icon's image file created from the cutbrush, is saved using some auto-generated hexadecimal name.

Even letting the user rename that file but keep its association with the button would be a great help in sorting out icons and the buttons they are assigned to.

Sven