How to save image in laravel pivot table additional field

Question

I have belongsToMany relation with one additional pivot field

Model

class Submission extends Model
{  

    public function items()
    {
        return $this->belongsToMany('App\Item')->withPivot('amount');
    }     
}

class Item extends Model
{
    public function Submissions()
    {
        return $this->belongsToMany('App\Submission')->withPivot('amount');
    } 
}

Controller

public function store(Request $request)
    {
        $submission = New Submission;
        $Submission->total_submission = $request->total_submission;
        $Submission->save(); 


        $sync_data = [];
        for ($i = 0; $i < count ($request->item_id); $i++)
        $sync_data[$request->item_id[$i]] = ['amount' => $request->amount[$i]];
        $Submission->items()->sync($sync_data);

        return redirect('submission');
     }

Now I want to add another field that will contain images. How to save that images to database.