I use photoshop usually. If you are looking for something free and easy to use, you can always use Canva.
I save mine out as a .jpg.
Here are the sizes I do for different things on Tapas:
- Cover - 960 x 1484 px
- Square Cover - 460 x 460 px
- Banner - 1280 x 460 px
- Footer at end of each chapter (asking to like and subscribe) - 1280 x 460 px