To create a cover, I obviously follow the rules in the last tip.

But adding onto it you need to be careful with how you edit the picture. Because unless you spent the hours upon hours it probably took to draw it, you have no right to try and claim it as your own. 

When using a picture, if there is an artist's watermark/signature you need to leave it be. 

I can't speak for the artists, but when I use a piece of fanart, I try to leave it as original as possible. So original that most of the time it's just the text added to it. 

However, I don't think there would be an issue applying a type of filter to it to brighten the image. I have done this before, as a way to add emphasis to the image or text. But remember that is still changing the image, and you need to stay respectful of the original artist. 

When searching for a cover I usually end up finding like 5 - 10 images that I could use. Just to give me some variety, and to play around with the text and see which I like best,

When editing, you can use Wattpad Covers. It's an app for your phone, and you can edit there and then immediately draw it from the Wattpad app. No need to download to upload again. 

However I prefer an app called Pixlr, it's just got more editing options and I also like to do photoshop every now and again with it as well, so I just stick with it.