okay, let's start with the basics. although every other cover tips book has this covered, (that pun. . .i'll see myself out) this is the main thing that you need for covers. these are all for Apple or Android (I think) devices.

number one - Wattpad Covers

I'm sure at one point that we have used this app at least once, whether to crop the graphic or to do the basics like add text and such. Now, I'm not saying that you shouldn't use this app to add text, but the only reason I use this is to crop something because, let's face it, there's not that many font options and you sorta have a hard time placing your text.

If you're a beginner, you might not really care if the graphic looks good or not. Listen to me now: do not use this app for adding fonts unless you are a straight-on professional at graphic making.

recommendation level:
blending: 0/10 (you can't blend at all)
fonts: 7/10
cropping: 10/10 (for covers only)

number two - PicsArt

For me, this is the best app I could ever use to make a graphic. I've been using it ever since I found out about it, and it's perfect for blending your graphic and works well for creating manipulated graphics. Many of you probably know about this and use it daily in your graphic-making life.

The more you use PicsArt, the more you get experienced in blending and making your graphic different from others. When I first started PicsArt, I only knew how to add pngs and small textures. Let's just say that I've come a long way from where I was back then. Go download it if the only graphic-making app you have is Wattpad Covers! Shoo!

ratings:
blending: 10/10 (it really depends if you are experienced.)
fonts: 8/10 (like I said with WC, it doesn't have many options. but if you download fonts there, then I guess it's okay.)
filters: 10/10 (the filters in PicsArt really help with your graphic. you can even change the colour of your models' eyes!)

number three - Phonto

Ah, the one and only Phonto. This app is perfect for fonts and all that jazz. There are lots of starter fonts that you could work well with, but you can download fonts as well! To be perfectly honest, I don't really use Phonto's starter fonts because they don't really match with most of my graphics.

Overall, Phonto is a really good app for fonts. There are some symbols that you can add too, but I prefer not to.

ratings:
fonts: 10/10 (it really depends if you like them or not or if you download fonts)
blending: 0/10 (well, you can't blend at all: it's a font app)
filters: 9.5 (the filters are really good but there aren't much unless you pay)

-Polarr is an app for filters and great if you want a certain colour scheme for your graphics. You can also download filters made by your fellow Polarrists.

-Superimpose is an app I've never tried before(but would like to), but because of high praise from fellow Wattpaders, I suggest trying it out. It's not free, but it's worth the price.

-VSCO is another filter app that I haven't tried yet.

-WeHeartIt is a great app for finding pictures for minimalistic or simple graphics.

-DeviantArt is another stunning app where you can find resources for your graphic-making needs, such as textures and pngs.
((my deviantart is highthrones))

Hope this helps!