how to use validation rules exists for image upload field in Laravel 5

i used laravel5

in Update,Create Request file

public function rules()
{
    return [
        'image' => 'required', 
        'description' => 'required', 
        'link' => 'required', 

    ];
}

but update request needs change image rule because if user modify record image is not require.

i think only not null image database, is Enough.

via Chebli Mohamed