@sorryforbeingaburden I actually use Photoshop but if you're asking for good apps, I'll suggest a few to you which I think are pretty amazing. ^^
1. Pixlr (pretty much is a combination of many filter/drawing/and many more apps)
2. LINE Camera (I usually use this for filter)
3. PhotoshopMix (yes this is an app on AppStore)
4. Write Behind (use this to type the text you want on the cover if you want to create something like how the character cover the title)
5. Phonto (for font, you can download more fonts on Dafont.com, I usually like using the font Bromello)
Hope that these are useful! :D
Lots of love,
Wifi