I'd assume it's legit. I'm not sure how anyone would fake the Staff tag on the message. That's a private DM sent to your Tapas Wall, right?
From reading the message, they mention if you choose to update images, include the new ones as attachments. So, I'd bet on someone having reported your cover art. I looked up your story and did a reverse image search on the cover, and I was able to find the artist on Art Station. They state it was a commission for you back in 2023. And their gallery looks legit. They even have a speed painting of your commission on the page. You could use that as proof that the art is, in fact, real and something you paid for.
That said, you've got a couple of things going on that you should consider fixing to prevent this in the future:
1. You don't have a banner on your story page. Since you commissioned such a high-end piece of art, it looks suspicious at a glance that you don't have a banner. It's something you see a lot with AI users; they'll have a gorgeous cover but then not put in the effort to polish the rest of their page. You should def think about making one, even if it's just your story's title or something. It'll help.
2. You also don't credit your cover artist. Putting a credit in your story description would instantly give people an artist to search for and remove any suspicion of whether the artwork is real or not.
