Why would it depend on request rate rather than upload rate? Just pre-scale several variants on upload. Then run an MR job every now and then and delete the variants that haven't been touched for more than e.g. a month. Scale those on demand, store recents and frequently accessed pre-scaled. This is literally a few days of work.
It's much nicer to be able to resize on demand. Many CDNs and image/file hosting services offer transform abilities via URL which makes it easy to get exactly what you need and optimize for every site and device.
But since images are immutable, this is a perfect use-case for lambda functions fronted by a CDN.
