On Wattpad there are quite some unique graphic types, but also things that are known outside of Wattpad. What's also important is graphic sizes.  You can go with the official size, or size it up for better quality. I'll describe every graphic type and put the size next to it.

Cover (Wattpad)
The original size for a Wattpad cover is 512x800 pixels. That makes the quality of the images rather bad, so most people choose to go with 1024x1600 pixels, like this. Covers are made to attract readers, a good cover can give you a lot of reads!

Icon

An icon, used as profile picture, is always a 1 by 1 ratio. This can be 500x500 pixels, 700x700 pixels, or any other amount of pixels, as long as the width and height are the same.

Banners
Banners, sometimes called breakers, are put in a story to show a time skip or change of perspective. They have the shape of a horizontal rectangle.

Blends
Blends are graphics where you... blend things. (Look at me go with my descriptions) But really. You take textures, overlays and a lot of pngs and make them look like something, at least, that's what I think. You know the concept of comfort zone to panic zone? Well, blends are my panic zone with graphic design. I have made one, for a contest. That's it. They are 800x600 pixels, or a multiple of that size.

Header
A header is a graphic of 1920x600 pixels, and put behind the icon on your profile. It's nice if it matches with your icon.

You have a lot of other graphic types, but they are less common. You have movie posters, pretty self-explanatory, bookmarks, what you won't use on Wattpad, unless you want to put a piece of paper in your phone (it's handy for when you have a professional request, of course.

Disclaimer:
If you find this book anywhere outside of Wattpad, don't read it. You will probably get malware on your device. This is a book on Wattpad, nowhere else. All rights go to me, on Wattpad. Don't read pirated works.