To migrate your existing images to Cloudinary, write a short script that traverses your images and upload them one-by-one using Cloudinary's upload API.
Cloudinary has client integration libraries for all major development languages which will simplify the call to our upload API. Find your client library here.
Are you hosting your existing images on Amazon's S3? notice that you can specify a remote HTTP or S3 URL of images as the file parameter of our upload API instead of sending the actual image data. This allows for a much faster migration process. If you want to specify remote S3 URLs, the S3 bucket should be whitelisted for your account. In order to set it up, please send us the name of the S3 bucket and the name of your Cloudinary account.
If you are using Ruby - Cloudinary's Ruby GEM includes a Migrator tool for managing the migration process for you: http://cloudinary.com/documentation/rails_additional_topics#migration
You might want to also look at our powerful Automatic-migration tool.