I've worked with systems that pre-size images on creation, and I'd much rather work with a system like this that resizes and caches on demand.
You never know when you're going to need a new size (the introduction of retina images a few years ago doubled the dimensions of images you need to serve for example, and your design team is likely to come up with new size requirements occasionally as well) and backfilling to resize millions of images is a big pain and costs a lot in terms of storage.
You never know when you're going to need a new size (the introduction of retina images a few years ago doubled the dimensions of images you need to serve for example, and your design team is likely to come up with new size requirements occasionally as well) and backfilling to resize millions of images is a big pain and costs a lot in terms of storage.