It depends on the load. If you’re expecting something in the realm of 100s images a day, doing it in your Elixir process should be fine. It’s simple and gives you the same workflow in dev and production.

If you’re expecting a large or bursty volume, then Lambda is a great way to solve the scalability problem. Note: For something like this, I’d have a simple python script on Lambda, not Elixir since it’s just shelling out to ImageMagick.

Overall, I don’t do any upfront transformations on images any more. Instead, they go up to an S3 bucket and go through a resizing proxy like Imgix or Thumbor. It’s a lifesaver when your designers change… well… everything, that’s what they do!