A great resource for creating cover images is Canva.com
You can use this website to create a unique cover image which has the resolution that works best with the Wattpad website!
Just sign up and you can start creating covers on your account.
There's even a section dedicated to Wattpad Covers if you select 'All Your Designs' then search for 'Wattpad Book Cover' (it might even be recommended in that section for you).
Try it out and if you have any questions, feel free to contact Canva Help.