Icons are pretty easy to set up. I would create an Iconset. This is a an xml file with some images in a .zip, renamed to .dis.
This file should be stored in folder %APPDATA%\GPSoftware\Directory Opus\Icons. There will be some examples in that folder, Copy the .dis files, and rename it to .zip.
You can find free icons here https://www.iconfinder.com/. Also this might help, but its possibly overly complex how-to-make-your-own-icon-packs.
Once you do this you can use your icons in buttons.