Yeah you may have heard the term, "Don't judge a book by its cover"
But sadly we all do.
It doesn't have to be a fancy cover, and if you want one there are many wonderful graphic designers on wattpad that are happy to make you a beautiful cover.
I guarantee that if you have a cover on your story, you will get much more reads.
If you don't have a cover, your story seems sloppy, even if the writing isnt.
You will want a cover that fits your story. Try and use colors that match the genre, for example if it's a horror book than you will probably want dark creepy colors or red for blood.
If it's romance, you'll probably want lighter colors like pink and red.
You want a cover that hooks people's attention. I wouldn't just put an image for the cover because it doesn't really appeal to the eye, although it's better than nothing.