hello, im so sorry for the late reply; the answer is no, i was not able to get their permission for the fanart, and i apologize that you are being attacked on behalf of my negligence.
i've stated in my warnings when ordering is that i would be using copyrighted and non-copyrighted photos. that being said, i do my best to search for the original artists and add them into the credits aside from me as the book cover editor. if i wasnt able to provide you their name, then it might be because when i found it, there was no artist linked to the photo. that's my fault though for not providing/searching for it to be added to the credits list/book description. i admit my shortcomings on that.
additionally, I'm genuinely curious, I'd like to ask why was the book cover posted on DeviantArt for the case? from what i remember, i asked that all works/orders made by me is to be only used on Wattpad and for personal use? next time, please refrain from doing that, thank you.