If you're the translator and want to add one yourself, you can choose any graphic that is not copyrighted and add the novel's name (+ the author's) onto it and add it to NU. Apparently not.
then just a title card with name of the series or generic representation stolen from another series or a simple unrelated 6 digit number for academic purposes materials
which of the 3, i'm pretty sure the first one with just the title in a white backround is a valid option?
If you wanna add it as a fanart, no problem, but to put it as a cover is a no go unless you got the official one from the author
anything that falls under 'fanart' or fan-made are no go. a remastered, might be. if you holds a license for the translation, then your covers are considered 'official' and can be accepted, though not necessarily strictly so. for example, while webnovel (that webnovel) have 'official' cover for their english releases, the original (raw) covers might still be accepted (if the overlord likes it more). Spoiler: example in case https://www.novelupdates.com/series/lifetime-of-bliss-and-contentment-with-you/ vs https://www.webnovel.com/book/14761125405537605/Lifetime-of-Bliss-And-Contentment-With-You
The author allows me to translate, and he doesn't particularly care about what cover I use... Oh, by the way, can I use sexy cover? <- main point of the question
sexy cover- yes, nsfw- no. click here to start a pm with the overlord and detail your request to him. https://postimages.org/ is quite handy for temporary images hosting