For icons there might be a command you need to run to update the icon cache...

gtk-update-icon-cache I think???

If that works.. what you'll want to do is include a postinst script in your debian package that runs that command. The postinst will execute, as it's name implies, after the installation of the package automatically.

Or.. might be something as simple as making sure your path is the correct path to the system icon files?