1. Submit, Submit Art, Enter text or select file from sta.sh. You can select a file to upload but I've never used it. It may involve converting the file or something.
2. There are free tutorials on making nice book covers using Photoshop, Sai, Gimp and the like. If you really want someone else to create the cover, then you need to post in Projects. (You've posted this in the wrong forum anyway. You really do need to read the forum rules before posting)
The audience on Wattpad and the literature community on deviantart are vastly different. In general, the lit community on here aren't interested in flashy covers. As long as the writing is good, that's all they care about. While in someone's gallery or favourites, if you hover the cursor over literature, the thumb will preview the first few lines of the piece. If your writing has a cover, it'll replace the writing. This means that at first glance, it might not be obvious to viewers whether it's literature or whether it's digital art or the like. Something to keep in mind.
You dont really need art on a bookcover, you can always pick some nice font and stuff sadly I only use sai for painting but im pretty sure people use photoshop for that kind of stuff. BTW welcome to the forum 👌