How to access images under writable folder

The writable folder should be used for temporary and/or transient files. Everything inside should and can be deleted

That is not a given, and is up to the developer. The goal of the writable folder is to be writable. By convention files added here are exempt from repo storage, so it is also a good place to store user/environment/temp files, but there is no issue with keeping uploaded content in place there and serving it out via symlinks or a media reader.