Taken from http://tango.freedesktop.org/Tango_Icon_Library, version 0.8.90 / public domain:

- prefsFeatures.png (converted from preferences-system.svg)

https://openclipart.org/detail/2281/information-icons-set-by-kuba / public domain:

- information-icons-error.png
- information-icons-question.png
- information-icons-info.png
- information-icons-warning.png

https://openclipart.org/detail/171382/cross-icon-by-zawertun-171382 / public domain:

- close.png
