Cloudinary lets you specify a default_image parameter when using the cl_image_tag, along with all of your transformations, to provide a fallback image if one has not been uploaded. If, however, you are using CarrierWave, you’ll likely want to define your transformations and the fallback image just once in your Uploader class, but this is not well documented - here’s how to do it.

https://www.stormconsultancy.co.uk/blog/development/tips-tricks/setting-a-default-image-when-using-cloudinary-and-carrierwave-in-rails/