you might want to try one of the solutions to set a wallpaper from the old forum
and try it here in App Inventor
see for example this contribution by Peep https://groups.google.com/g/mitappinventortest/c/34cj32OmFq8/m/vlQEkQ8lpI4J
Use Activity Starter.
You can use those actions:
- Action: android.intent.action.SET_WALLPAPER
or
Action: android.intent.action.ATTACH_DATA
DataType: image/*
DataUri: file:///storage/emulated/0/dir/filename.jpg
It depend what kind app you make and where you get your data: etc. camera, internet pictures or allready stored pictures to app
Taifun