Do I need to upload anything to S3?
No. This module will take care of that for you. You don't need to upload any images to S3. Product Downloads are uploaded to S3 each time you save your Product Download in the back office or use the bulk upload tool.
Where do I upload the Private Key?
You need to upload your Private Key somewhere safe on the server where no one could access it. For starters, you could upload it out side your server's web root.
Why are my original images not on Amazon S3?
Normal product images are NOT uploaded or stored on Amazon S3, only the image cache files. Images have to be uploaded normally using OpenCart, and these original image files remain on the web server. Cached versions of these files are then saved and served from Amazon S3.