Again, it's not stealing, since it's still the work of the artist, unless I were to lie and say I was the one who created it, which would actually be a "shitty" thing to do. You're just using it in a way to supplement your writing. Asking for permission is irrelevant, since the use of the art is not taking away anything from the artist, nor is it belittling the work that they put into it, or causing any kind of damage.