Coolest would be an ability to have 1) a great filepicker UI (drag target + multi file upload w/ progress bar) 2) resizing/thumbnails in the browser 3) no server side other than s3
filepicker.io is cool. But your backend service wouldn't be needed if the image uploading & resizing straight to s3 could be done in the browser. Your product would simply be a javascript lib.
guessing #2 isn't possible?