Mentioned are the folders /iv-config/images/icons/ and /iv-config/icons/, but neither of them will work yet.
The custom icons do work, though, when they are located in /iv-includes/images/icons/.
Furthermore, the text there says:
But this also has to be: /iv-includes/images/icons/.We have already included a selection of icons in the iv-config/icons/ folder that you can use in your menu as desired.
Anyway, I like this possibility of applying custom made icons!
I used it to have the flag of a country in the folder that's being used for language selection.