When storing images to S3 with Assets 2, is it possible to store the images created by ExpressionEngine's native image manipulation functionality to S3 as well, or must they be saved to a local directory?
The Image Manipulations with EE are directly tied to a File Upload preference - which is a local source item, not an S3 bucket. I don't think that what you want is currently possible.
But, what I hear you saying is that you wish to use image manipulations and have those modified images uploaded/stored/referenced via S3. Is that right?