NivoSlider 5 images?

Make sure you uninstall your original Nivo Slider in the admin area.

Then replace with you new modified one > reload plugins, you should see six.

I think it should be changed and controllers and model file

follow this post for NopCommerce 3.5

https://www.nopcommerce.com/boards/t/36285/nopcommerce-35-nivo-slider-more-than-5-images.aspx