Both yes and no:
Yes, you may use our art in accordance with the GPL2 or later license and the conditions stated there. Summed up it means that the project you create from our art or the project that incorporates our art, will also become open source and licensed using the GPL2 or later. That is a great power, and with great power comes great responsibility. If you only use our original artworks (and not the BfW works) you may also/instead use the CC BY-SA 4.0 license.
You do not need to ask us or tell us that you are doing it. It is all totally up to you. If you do use it though we would still appreciate a the info from you even if you are not legally obliged to contact us. That way we can feature you in an appropriate place for exposure.
You can’t use our art if you want to include it in anything that is closed source and/or using any other license than what is described above.