Varies from client to client. Amethyst seems to use imgur.
I've seen some calls to use IPFS, but that has its own incentive problems.
TL;DR if you want aligned incentives then you should probably host your own images.
Discussion
We should at least use a content hash based filename convention so the same file could be looked up from different hosts.
Nostr.build already does hash names:
https://nostr.build/i/ nostr.build_57d7f84a0d51fb6bfe97ad288dd6f54d1747ab2a915da56b8e2966a77e9f19bc.png.
But I think bech32 "file1234asdf..." would be even better.