Yes, I can use any software to upload to S3 instead of Jalbum. Anyway, I was wonder if JAlbum planning to implement it because it would be easier to use it the same way as FTP.
And no, AWS does not support FTP protocol without 3rd party service.
Whatever that means, they are claiming:
Getting started with AWS Transfer for SFTP (AWS SFTP) is simple. Create an SFTP server and map your domain to the server endpoint, select authentication for your SFTP clients using service-managed identities, or integrate your own identity provider, and select your Amazon S3 buckets to store the transferred data. Your existing users can continue to operate with their existing SFTP clients or applications. Data uploaded or downloaded using SFTP is available in your Amazon S3 bucket, and can be used for archiving or processing in AWS.