In this Fatal Error App\Http\Controllers\ not found the laravel wants to show that the model you want to use that is not available in your scope.
What does it mean thal model is not in scope for laravel ?
Laravel searching for the model class which is not included in page/controller.
So before use any model in controller you have to include or use that into your controller.
Note:-Above given error is very generic. It is not only for model.
it can be for session,file system ,encryption,database and many more .
So whenever this kind of error is occurred for session,database,file and so on ,than simple is that you have not included that class which you wants to use.
here wallpaper is our table name or model name, we are trying to use that in our controller but we have not included that
so just add :-
use App\wallpaper; // or whatever your model location
into your working controller file