//composer  require intervention/image 
$img = Image::make(public_path('image.jpg')); //your image I assume you have in public directory
$img->insert(public_path('watermark.jpg'), 'top-right', 10, 10); //insert watermark in (also from public_directory)
$img->save(public_path('image.jpg')); //save created image (will override old image)