how to build an deep learning image processing server
It does make sense to look at the whole task and how it fits to your actual server, Nginx or Lighttpd or Apache since you are serving static content.  If you are going to call a library to create the static content, the integration of your library to your web framework would be simpler if you use Flask but it might be a fit for AWS S3 and Lambda services.
It may be worth it to roughly design the whole site and match your content to the tools at hand.
I am building am application to process user's photo on server. Basically, user upload a photo to the server and do some filtering processing using deep learning model. Once it's done filter, user can download the new photo. The filter program is based on the deep learning algorithm, using torch framework, it runs on python/lua. I currently run this filter code on my local ubuntu machine. Just wonder how to turn this into a web service. I have 0 server side knowledge, I did some research, maybe I should use flask or tornado, or other architecture?