It depends on the type of cover you wanna make…if it’s an edited one then some apps you can use are Picsart, Pixelab, Gimp and AfterFocus. Picsart has a number of filters, stickers and plenty of tools. For the text, I’d recommend installing pixelab as it can help you to add shadow-ed, curved, italized, gradient or any type of text you wanna add to the cover. Gimp- for compressing or editing the cover and finally AfterFocus- If you gonna blur some parts of the cover. All you need to do to make an edited cover is to download the picture of the characters in their respective poses and outfits and paste and edit them on the background with the above-listed apps.
Now, if it’s gonna be drawn, some apps I recommend are Procreate and lbis paint x.
Lbis paint is an app with amazing user interface and structure. It is very simple and easy to use. Also there are 100+ brushes and unlimited no. of layers you can use in the app. It also comes with a number of filter including blurs. I use the app myself to draw and too he honest, it’s a feally amazing app except for the adds which make the canvas somewhat smaller and are sometimes annoying . Just add a picture of the characters in their specific poses on a layer, add a layer and then draw the outline of the char. Again, add another layer and color the characters. You can delete the reference pic or disable it now if you wish.
Procreate- The name in itself explains the functionality of the app. But I’ve never used this app so I’m just gonna say what I’ve heard from other artists…the app comes with a very wide range of brushes. It’s a very intuitive app, with powerful features for doing excellent illustration work. Procreate offers exceptional performance, an elegant user interface, powerful layers support, stunning filters, hundreds of brush presets (including pens, pencils, and abstract tools), and the ability to import, create, and share custom brushes.
But as you see, the app isn’t free, also it’s available only on ipads.
If you need more tips on drawn art, here’s the link
That’s all I’d like to say. Hope it helps.