High volume photo resizing - best approach

Hi, does anyone have experience with or could recommend:

  1. The best approach to build an app for uploading and resizing a high
    volume of photos contributed by users.

  2. An uploader to allow users to upload multiple photos at a time vs
    one at a time.

  3. Best image processor. I’ve been told by a friend that Mini Magick
    performs best under high load, would be interested in any real world
    feedback.

  4. Best cdns. Any tips on best cdn to serve images also much
    appreciated and if there’s any gems that already integrate with them
    for easier transfer.

Thanks,
Pardeep.