DraStic

i would just go for the resolution of your phone and call it good no need to have a over resolution photo as a background image for me personally i wouldn't do anything over 1920x1080p, i mean the 3ds is max 800 x 240p so... yeah. I personally think 4k phones are meh no point on a 5inch screen...

just a suggestion if you do go to free and paid as someone stated make the current app "gold" and with the free version take some things out like...

cheats...( or the database part of it limit custom cheats)
google cloud sync... ( if you feel this is harsh just lock out the automatic function of it ...