Not sure if someone has filed this issue as an 'official' bug but sure it has been reported elsewhere on the forum.
In the new Gnome version 2.28 and later, menus no longer have icons by default. PixInsight 1.6.9 has been respecting this criterion because Qt 4.6 and 4.7 also do. Since many Linux users have complained about the change, and we in fact also prefer menus with icons (and as far as I know, most Linux applications don't adhere to Gnome's policy), we have decided to revert PixInsight to the 'old' behavior: menus always have icons on all platforms. This will be effective in the next version, which will be made available as an update very soon.
In the new Gnome version 2.28 and later, menus no longer have icons by default. PixInsight 1.6.9 has been respecting this criterion because Qt 4.6 and 4.7 also do. Since many Linux users have complained about the change, and we in fact also prefer menus with icons (and as far as I know, most Linux applications don't adhere to Gnome's policy), we have decided to revert PixInsight to the 'old' behavior: menus always have icons on all platforms. This will be effective in the next version, which will be made available as an update very soon.