Found a way to change it on android,
1. Download Z Archiver
2. Go to storage folder
3. Go to emulated folder
4. go to folder called zero
5. Go to Android folder
6. Go to data folder
7. Swipe down to folder called wolf.complex
8. Go into wolf.complex folder and then into files folder
9. Tap the plus sign in the bottom right
10. Make a folder called wolfcomplex/game/custom_background.png
11. Pick image you want cut it from the downloads folder and bring it into wolf.complex
12. Rename it custom_background.png
13. Paste it into game folder it will give you a prompt tap rename then delete custom_background.png folder
14. Rename the image back to custom_background.png
15. Cut the game folder out of the folder wolfcomplex
16. Paste it into files folder
17. Delete the now empty folder called wolfcomplex
18. Hurray the image in the android version of the ame is changed
19. If you want to change the image again you have to go through all these steps again or it gets upset for some reason
20. Sorry for the many steps hope this helps