Seriously? You want a designer to design your book cover in exchange for a credit line? When you visit your dentist or hire an electrician, do you tell them you’ll pay for the work in exchange for a social media mention?
People here make their living as graphic designers. A credit line doesn’t buy food or pay the rent. You’ve outlined a project that would involve research, multiple mock-ups, back-and-forth conversations, a series of revisions, and eat up several days’ worth of time.
On top of that, I won’t even touch a book cover without first quickly reading the book to make sure the cover accurately conveys the book’s personality and contents. I also wouldn’t touch a job for a naive client who assumes that I work for free instead of a couple of thousand dollars in advance.
If you want cheap work, head to one of the crowdsourcing sites and hire an amateur to design your book cover for a couple of hundred dollars. No one but a fool would design a book for free, and even if you managed to hire a fool, you would get a book cover that was worth exactly what you paid for it.
In addition, you apparently didn’t read the forum rules, which specifically prohibit asking for free work.