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Amazon S3 is an inexpensive way to store an arbitrary amount of data in completely safe, robust, fast storage. If you have a lot of images, movies, podcasts, or other large data files, you should use S3 as the storage engine rather than uploading and serving files directly from WPEngine.

There’s a free WordPress plugin you can use that transparently stores your images on S3 instead of the local drive:

http://WordPress.org/extend/plugins/tantan-s3/

Here’s a good tutorial on how to set all that up:

http://www.labnol.org/internet/host-images-files-on-amazon-s3-storage/4923/