For uploading images to the cloud, you need to be online of course. Same goes for transformed image delivery. However, all transformation URL building is performed on the client side using our client libraries and you can be offline for this to work.
In addition, in certain development environment, you can disable the actual image uploading.
In PHP for example, you can avoid the actual uploading (use your custom environment based conditions) by returning a dummy public ID on your side without uploading to Cloudinary. Then use our image transformation URL building from your PHP testing code. No server access is required. All methods in the following links are client-side (offline):https://github.com/cloudinary/cloudinary_php#embedding-and-transforming-images .
For CarrierWave there is a testing flag that can be used.