as I said use relationship for saving account and delete user_id from fillable and

 <form method="POST" class="form-inline" enctype="multipart/form-data" action="/store">
           {{ csrf_field() }} 
              <div class="form-group">
              <input name="avatar" class="form-control" type="file" required="" />
              </div>
              <div class="form-group">
               <button type="submit" class="btn btn-primary pull-right">Submit</button>
              </div>
            </form>

controller

public function update_avatar(Request $request)
    {
         $this->validate($request,[

            'avatar' => 'required|image|max:10240',

        ]);

         $image = new Image;  
  //change model name Image to something else because u are using Image as alias  if u not then it will give u a errors 
        
      $image->user_id = Auth::user()->id;


        if ($request->hasFile('avatar'))
         {
            $avatar = $request->file('avatar');
            $filename = time().'.'.$avatar->getClientOriginalExtension();
            Image::make($avatar)->resize(300,300)->save(public_path('image/'.$filename));

            $image->filePath=$filename
            $image->save();
        }
          //flash()->success('Successfully Profile Pic. Updated');

          return back();
    }